CN110212505B - Method for selecting current-limiting reactance of flexible direct-current transmission system based on superconducting current limiter - Google Patents

Method for selecting current-limiting reactance of flexible direct-current transmission system based on superconducting current limiter Download PDF

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CN110212505B
CN110212505B CN201910449385.2A CN201910449385A CN110212505B CN 110212505 B CN110212505 B CN 110212505B CN 201910449385 A CN201910449385 A CN 201910449385A CN 110212505 B CN110212505 B CN 110212505B
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current
direct current
transmission system
flexible direct
fault
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CN110212505A (en
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郭佩乾
袁志昌
陆超
余昕越
魏应冬
屈鲁
余占清
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Tsinghua University
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    • 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
    • H02H9/023—Current limitation using superconducting elements
    • 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
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    • 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
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Abstract

The invention relates to a method for selecting a current-limiting reactance of a flexible direct-current transmission system based on a superconducting current limiter, and belongs to the technical field of direct-current transmission. The method aims at solving the problem that when a direct-current side line fault occurs in a modular multilevel flexible direct-current transmission system, the fault current is restrained by calculating the current-limiting reactance of the direct-current line and matching the current-limiting reactance with a superconducting current limiter, so that the multilevel converter is guaranteed to finish fault clearing under the condition of no locking. The method improves a substitution calculation method for selecting fixed impedance, and optimizes the solution by piecewise linearization fitting of the resistance value of the superconducting current limiter based on a least square method. The method can calculate the reasonable value margin of the current-limiting reactance under the influence of the dynamic resistance value of the superconducting current limiter, effectively improves the stability and the dynamic response capability of the system, and has guiding and reference significance for future practical engineering. The method comprehensively considers the matching among various primary equipment parameters, selects the primary equipment more reasonably, improves the utilization efficiency of the primary equipment, reduces the system investment cost and improves the economy.

Description

Method for selecting current-limiting reactance of flexible direct-current transmission system based on superconducting current limiter
Technical Field
The invention relates to a method for selecting a current-limiting reactance of a flexible direct-current transmission system based on a superconducting current limiter, and belongs to the technical field of direct-current transmission.
Background
The flexible direct current transmission system (VSC-HVDC) has the advantages of not considering the frequency and phase problems of the connected alternating current system, having small interference and influence between the interconnected systems, being capable of quickly and accurately controlling the transmission power and the like, and effectively improving the stability of the system. However, compared with an ac system, the flexible dc system has a very fast speed of short-circuit current when a fault occurs due to its low impedance, and the fault clearing difficulty of the flexible dc system increases as the fault current of the flexible dc system does not have a natural zero crossing point.
From the perspective of improving the power supply reliability of the direct current transmission network, the direct current circuit breaker is used for fault isolation, so that a selective fault isolation function in a minimum range can be effectively realized. However, in view of the development bottlenecks of the current dc crowbar, such as the breaking speed and breaking capacity, the reduction of the operation and capacity requirements of the dc breaker based on the related current limiting technology is required for future development. Through analysis, the fault current can be effectively inhibited by assembling the current-limiting reactance at the side of the direct current line, and the dynamic characteristic of the direct current fault current is greatly influenced. However, the main contradiction of this solution is focused on the inductance parameter selection, and considering the stability of the system, the dynamic response speed, the grid structure and other factors, an excessive current-limiting impedance cannot be installed in the dc power transmission network, so the current-limiting capability is relatively limited.
The Superconducting Current limiter (SFC L, Superconducting Fault Current L timer) can effectively inhibit the amplitude and the spreading speed of Fault Current and enter the sight of researcher due to the resistance value characteristic changing under different states by matching with a direct Current side Current limiting reactor.
Disclosure of Invention
The invention aims to provide a method for selecting a current-limiting reactance of a flexible direct-current transmission system based on a superconducting current limiter, which is used for calculating a reasonable current-limiting reactance range in the flexible direct-current system considering the dynamic change impedance value of the superconducting current limiter so as to improve the response capability and stability of the system.
The invention provides a current-limiting reactance selection method of a flexible direct-current transmission system based on a superconducting current limiter, which comprises the following steps:
(1) constructing a double-ended modular multilevel flexible DC power transmission system with a superconducting current limiter, the double-ended flexible DC power transmission system comprising a first AC line simulated impedance Z1The second AC line analog impedance Z2First modular multi-level flexible direct current converter MMC1Second modular multi-level flexible direct current converter MMC2First direct current breaker DCB1And a second DC breaker DCB2And a third DC breaker DCB3And a fourth DC breaker DCB4A first current limiting reactor Ld1A second current limiting reactor Ld2A third current limiting reactor Ld3A fourth current limiting reactor Ld4And a superconducting current limiter; the first modular multilevel flexible direct current converter MMC1Inlet end and second modular multi-level flexible direct current converter MMC2Respectively through a first AC line to simulate an impedance Z1And a second AC line analogue impedance Z2To the alternating current network AC1And the alternating current network AC2Connected, a first current limiting reactor Ld1And a second current limiting reactor Ld2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1And a third current limiting reactor Ld3And a fourth current limiting reactor Ld4Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2Inlet end of, a first dc breaker DCB1And a second DC breaker DCB2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1Of the third direct current breaker DCB2And a fourth DC breaker DCB2Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2The superconducting current limiter SFC L is arranged on a negative direct current circuit of the double-end flexible direct current transmission system and is positioned on a second direct current breaker DCB2And a second DC breaker DCB2To (c) to (d);
(2) by using the following formula, the dynamic impedance R of the superconducting current limiter when the DC line interelectrode fault occurs at the outlet end of the superconducting current limiter SFC L in the double-end flexible DC power transmission system is calculateds(t)：
Figure BDA0002074644060000021
Wherein R ismWhen the superconducting current limiter is connected into a double-end modular multi-level flexible direct current transmission system, the maximum resistance value T of the superconducting current limiter in a quench statesThe state time constant of the superconducting current limiter is converted from a superconducting state to a quench state and is obtained by a nameplate of the superconducting current limiter, t is the operation time of the double-end flexible direct current transmission system, tqThe moment when the critical current flows through the superconducting current limiter;
(3) utilizing a least square fitting method to obtain the dynamic impedance R of the superconducting current limiter in the step (2)s(t) is piecewise linearized into n first-order impedance functions R 'varying with time t's(t)：
R′s(t)＝Ri(ti)(i＝1,2,3,…,n)
(4) MMC (modular multilevel flexible direct current converter) according to first modular multilevel1The number N of submodules and the capacitance C of the submodules0Calculating the equivalent capacitance C' of a fault loop when the double-end modular multilevel flexible direct current transmission system has an inter-pole fault of a direct current line by using the following formula:
Figure BDA0002074644060000031
wherein N is a first modular multi-level flexible direct current converter MMC1Number of sub-modules SM, C0The capacitance size in a single modular multilevel sub-module SM is obtained;
(5) calculating the equivalent inductance L' of a fault loop of the power transmission system when the double-end modular multilevel flexible direct current power transmission system has an inter-pole fault of a direct current line according to the following formula:
Figure BDA0002074644060000032
wherein, LsBridge arm inductor L passing through MMC1 of first modular multilevel converter for short-circuit currentlDC line reactance for double-ended modular multilevel flexible DC transmission system, LdA current-limiting reactance of a direct-current line of a double-end modular multilevel flexible direct-current transmission system to be solved;
(6) the fault loop equivalent resistance R' is calculated using the following equation:
Figure BDA0002074644060000033
wherein R isarmIs a bridge arm resistance R of a first modular multilevel flexible direct current converter MMC1 in a double-end modular multilevel flexible direct current transmission systemlIs a direct current line resistance R in a double-end modular multi-level flexible direct current transmission systemfFor fault resistance when a double-ended modular multilevel flexible DC power transmission system has an interpolar fault in the DC lines, R1(t1) Impedance of the superconducting current limiter in a quench state when the double-end modular multilevel flexible direct current transmission system fails;
(7) according to the equivalent capacitance C ' of the step (4), the equivalent inductance L ' of the fault loop of the step (5), the equivalent resistance R ' of the fault loop of the step (6) and the equivalent capacitance voltage v of the fault loopcAnd obtaining a stage mathematical expression when the double-end modular multilevel flexible direct-current transmission system has an interpolar fault of a direct-current line, wherein the stage mathematical expression is as follows:
Figure BDA0002074644060000034
(8) according to the stage mathematical expression in the step (7), calculating the time t when the inter-electrode fault of the direct current line occurs in the double-end modular multi-level flexible direct current transmission system by using the following formula1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current ofi1：
Figure BDA0002074644060000035
Where σ is the damping coefficient, σ ═ R '/2L ', ω ' is the oscillation angular frequency,
Figure BDA0002074644060000036
(9) setting the transmission power balance of an alternating current power grid and a modular multilevel converter when the double-end modular multilevel flexible direct current transmission system has interpolar faults of a direct current line, and obtaining the alternating current i of the double-end modular multilevel flexible direct current transmission system according to the following formulap：
Figure BDA0002074644060000041
Wherein, VdcDC voltage, I, for a double-ended modular multilevel flexible DC transmission systemdDirect current, V, for a double-ended modular multilevel flexible direct current transmission systemlThe voltage of an alternating current network line of the double-end modular multi-level flexible direct current transmission system;
(10) according to step (8) fault current i1And the alternating current i of step (9)pAnd calculating to obtain a first modular multilevel flexible direct current converter MMC in the double-end modular multilevel flexible direct current transmission system by using the following formula1At t at superconducting current limiter impedance1MMC (modular multilevel flexible direct current converter) capable of flowing through first modular multilevel in stage1Fault current i of bridge arm2：
Figure BDA0002074644060000042
(11) When a double-end modular multi-level flexible direct current transmission system has an inter-pole fault of a direct current line, a first direct current breaker DCB is arranged to flow through1And a second DC breaker DCB2All fault currents of are ibAnd parallel double-end modular multilevelWhen the flexible direct current transmission system has an interelectrode fault of a direct current line, the first direct current breaker DCB1And a second DC breaker DCB2Before action, a first modular multi-level flexible direct current converter MMC1Bridge arm current allowable limit value of iaAt t, in the event of a fault between the DC lines in step (8)1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1And (9) calculating the alternating current i of the double-end modularized multi-level flexible direct current transmission systempAnd the step (10) of flowing through a first modular multilevel flexible direct current converter MMC1Fault current i of bridge arm2Obtaining a first modular multilevel converter MMC in the case of interpolar fault of the direct-current line1The boundary conditions before occlusion are:
Figure BDA0002074644060000043
(12) locking a front limit fixed condition by using the first modular multilevel flexible direct current converter MMC1 in the step (11), and according to the equivalent capacitor C ', the equivalent inductor L ', the equivalent resistor R ' in the step (4) to the step (6) and the inter-pole fault of the direct current line in the step (8) to the step (10) at t1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1AC current ipAnd a first modularized multi-level flexible direct current converter MMC flows through1Fault current i of bridge arm2And matching the superconducting current limiter at t when the double-end modular multilevel flexible direct current transmission system fails through calculation1In-stage DC line with current limiting reactance LdThe value range of (a);
(13) limiting reactance L according to step (12)dBy the current-limiting reactance LdThe maximum value in the value range is used as the current-limiting reactance of the direct-current line when the double-end modular multilevel flexible direct-current power transmission system has an inter-electrode fault of the direct-current line;
(14) traversing the n segments of the superconducting current limiter impedance linearization in the step (3) and the corresponding first orderImpedance function R'sAnd (t) repeating the steps (8) to (13), and calculating to obtain the value range of the current-limiting reactance of the direct-current line when the double-end modular multi-level flexible direct-current power transmission system fails, so that the modular multi-level converter MMC can be effectively protected under the condition of no locking when the double-end modular multi-level flexible direct-current power transmission system has an inter-electrode direct-current line fault.
The method for selecting the current-limiting reactance of the flexible direct-current transmission system based on the superconducting current limiter has the characteristics and advantages that:
1. the method can calculate the reasonable value margin of the current-limiting reactance under the influence of the dynamic resistance value of the superconducting current limiter, effectively improves the stability and the dynamic response capability of the flexible direct current transmission system, and has guiding and reference significance for future practical engineering.
2. The method comprehensively considers the matching among various primary equipment parameters, selects the primary equipment more reasonably, improves the utilization efficiency of the primary equipment, reduces the investment cost of the flexible direct current transmission system, and improves the running economy of the transmission system.
3. The method for selecting the current-limiting reactance of the flexible direct-current transmission system based on the superconducting current limiter is not only suitable for installing the superconducting current limiter on a single pole of a double-end flexible direct-current transmission system, but also suitable for installing the superconducting current limiter on a double pole of the double-end flexible direct-current transmission system simultaneously.
4. The method for selecting the current-limiting reactance of the flexible direct-current transmission system based on the superconducting current limiter is not only suitable for installing the superconducting current limiter on a double-end flexible direct-current transmission system, but also suitable for installing the superconducting current limiter on a multi-end flexible direct-current transmission system; the superconducting current limiter can be taken as a resistive superconducting current limiter and can also be taken as other types of superconducting current limiters.
5. According to the method for selecting the current-limiting reactance of the flexible direct-current transmission system based on the superconducting current limiter, linear fitting can be achieved through a least square method, and other linear fitting methods are also applicable.
Drawings
Fig. 1 is a structural diagram of a double-end flexible direct current system related to the method of the invention.
Fig. 2 is a schematic diagram of impedance piecewise linearization of a superconducting current limiter according to the present invention.
Fig. 3 is a fault current path diagram before locking of a first modular multilevel flexible dc converter MMC1 according to the present invention.
Fig. 4 is a simplified equivalent diagram of a discharge circuit of a double-ended modular multilevel flexible direct current transmission system according to the invention.
Detailed Description
The invention provides a current-limiting reactance selection method of a flexible direct-current transmission system based on a superconducting current limiter, which comprises the following steps:
(1) constructing a double-ended modular multilevel flexible direct current transmission system with a superconducting current limiter, the structure of which is shown in figure 1, the double-ended flexible direct current transmission system comprises a first alternating current line analog impedance Z1The second AC line analog impedance Z2First modular multi-level flexible direct current converter MMC1Second modular multi-level flexible direct current converter MMC2First direct current breaker DCB1And a second DC breaker DCB2And a third DC breaker DCB3And a fourth DC breaker DCB4A first current limiting reactor Ld1A second current limiting reactor Ld2A third current limiting reactor Ld3A fourth current limiting reactor Ld4And a superconducting current limiter; the first modular multilevel flexible direct current converter MMC1Inlet end and second modular multi-level flexible direct current converter MMC2Respectively through a first AC line to simulate an impedance Z1And a second AC line analogue impedance Z2To the alternating current network AC1And the alternating current network AC2Connected, a first current limiting reactor Ld1And a second current limiting reactor Ld2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1And a third current limiting reactor Ld3And a fourth current limiting reactor Ld4Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2Inlet end of, a first dc breaker DCB1And a second DC breaker DCB2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1Of the third direct current breaker DCB2And a fourth DC breaker DCB2Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2The superconducting current limiter SFC L is arranged on a negative direct current circuit of the double-end flexible direct current transmission system and is positioned on a second direct current breaker DCB2And a second DC breaker DCB2To (c) to (d);
(2) by using the following formula, the dynamic impedance R of the superconducting current limiter when an inter-pole fault of a direct current line occurs at the outlet end (at FA shown in FIG. 1) of the superconducting current limiter SFC L in the double-end flexible direct current transmission system is calculateds(t)：
Figure BDA0002074644060000061
Wherein R ismWhen the superconducting current limiter is connected into a double-end modular multi-level flexible direct current transmission system, the maximum resistance value T of the superconducting current limiter in a quench statesThe state time constant of the superconducting current limiter is converted from a superconducting state to a quench state and is obtained by a nameplate of the superconducting current limiter, t is the operation time of the double-end flexible direct current transmission system, tqThe moment when the critical current flows through the superconducting current limiter;
the method considers the fault time of the double-end flexible direct current transmission system and the resistance value change condition of the superconducting current limiter under the quench state, so that the resistance value of the flexible direct current transmission system is not considered to be in the zero resistance value characteristic under the normal running state, namely R is ignoreds(t) is 0.
(3) Utilizing a least square fitting method to obtain the dynamic impedance R of the superconducting current limiter in the step (2)s(t) is piecewise linearized into n first-order impedance functions R 'varying with time t's(t)：
R′s(t)＝Ri(ti)(i＝1,2,3,…,n)
As shown in fig. 2, the solid line part is a resistance characteristic curve of the actual superconducting current limiter in the quench state, and the dotted line is a linear resistance characteristic curve of the superconducting current limiter in the quench state after the least square method based piecewise fitting.
(4) When a direct-current line interelectrode fault occurs at the outlet end of the superconducting current limiter SFC L between the first modular multilevel flexible direct-current converter station MMC1 and the second modular multilevel flexible direct-current converter station MMC2 and in the double-end modular multilevel flexible direct-current transmission system with the superconducting current limiter, at the moment, the capacitance of a submodule SM in the first modular multilevel converter MMC1, the bridge arm inductance L s in the first modular multilevel converter MMC1 and the bridge arm resistance R in the first modular multilevel converter MMC1armLine resistance Rl of the flexible direct current transmission system, self reactance L l of the direct current line of the flexible direct current transmission system, current limiting reactance L d of the direct current line of the flexible direct current transmission system, fault resistance Rf and resistance R of the superconducting current limiter during fault of the flexible direct current transmission systems(t) are connected in series to form a fault loop.
First modular multilevel flexible direct current converter MMC according to fig. 31The number N of submodules and the capacitance C of the submodules0Calculating the equivalent capacitance C' of a fault loop when the double-end modular multilevel flexible direct current transmission system has an inter-pole fault of a direct current line by using the following formula:
Figure BDA0002074644060000071
wherein N is a first modular multi-level flexible direct current converter MMC1The number of the sub-modules SM in the method is that a first modular multilevel flexible direct current converter MMC is used1MMC for example in the near zone2Structural parameters and MMC1Which may be the same or different, C0The capacitance size in a single modular multilevel sub-module SM is obtained; fig. 3 is a fault current path diagram before locking of a first modular multilevel flexible dc converter MMC1, and fig. 4 is a simplified equivalent diagram of a discharge loop of a double-ended modular multilevel flexible dc power transmission system.
(5) Calculating the equivalent inductance L' of a fault loop of the power transmission system when the double-end modular multilevel flexible direct current power transmission system has an inter-pole fault of a direct current line according to the following formula:
Figure BDA0002074644060000072
wherein, LsBridge arm inductance of the first modular multilevel converter MMC1 for short-circuit current (bridge arm inductance is the first modular multilevel converter MMC)1Partial elements, once the first modular multilevel converter is determined, the inductance is also determined), LlL DC line reactance for double-ended modular multilevel flexible DC power transmission system (the reactance is determined by DC line length)dA current-limiting reactance of a direct-current line of a double-end modular multilevel flexible direct-current transmission system to be solved;
(6) the fault loop equivalent resistance R' is calculated using the following equation:
Figure BDA0002074644060000073
wherein R isarmBridge arm resistance (MMC) of MMC1 as a first modular multilevel converter in a double-ended modular multilevel flexible DC power transmission system1Once determined, the bridge arm resistance is also determined), RlFor a direct current line resistance in a two-terminal modular multilevel flexible direct current power transmission system (as shown in fig. 3, the line resistance is provided by a first modular multilevel converter, MMC1And a second modular multi-level flexible direct current converter MMC2Length of the direct current line in between), RfThe fault resistance is fault resistance when a double-end modular multi-level flexible direct current transmission system generates an inter-pole fault of a direct current line (the resistance is caused by the fact that the line and the ground are included or the fault line is abnormally connected with each other when the fault occurs), R1(t1) Impedance of the superconducting current limiter in a quench state when the double-end modular multilevel flexible direct current transmission system fails;
(7) according to the equivalent capacitance C ' of the step (4), the equivalent inductance L ' of the fault loop of the step (5), the equivalent resistance R ' of the fault loop of the step (6) and the equivalent capacitance voltage v of the fault loopcObtaining the direct current appearing in the double-end modular multilevel flexible direct current transmission systemThe stage mathematical expression when the flow line interpolar fault occurs is as follows:
Figure BDA0002074644060000081
the submodule discharging equivalent circuit of the double-end modular multilevel flexible direct-current transmission system is shown in figure 4.
(8) According to the stage mathematical expression in the step (7), calculating the time t when the inter-electrode fault of the direct current line occurs in the double-end modular multi-level flexible direct current transmission system by using the following formula1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1：
Figure BDA0002074644060000082
Where σ is the damping coefficient, σ ═ R '/2L ', ω ' is the oscillation angular frequency,
Figure BDA0002074644060000083
(9) setting the transmission power balance of an alternating current power grid and a modular multilevel converter when the double-end modular multilevel flexible direct current transmission system has interpolar faults of a direct current line, and obtaining the alternating current i of the double-end modular multilevel flexible direct current transmission system according to the following formulap：
Figure BDA0002074644060000084
Wherein, VdcDC voltage, I, for a double-ended modular multilevel flexible DC transmission systemdDirect current, V, for a double-ended modular multilevel flexible direct current transmission systemlThe voltage of an alternating current network line of the double-end modular multi-level flexible direct current transmission system;
(10) according to step (8) fault current i1And the alternating current i of step (9)pAnd calculating to obtain a first voltage in the double-end modular multi-level flexible direct current transmission system by using the following formulaModular multi-level flexible direct current converter MMC1At t at superconducting current limiter impedance1MMC (modular multilevel flexible direct current converter) capable of flowing through first modular multilevel in stage1Fault current i of bridge arm2：
Figure BDA0002074644060000085
(11) When a direct-current line interelectrode fault occurs in a double-end modular multilevel flexible direct-current transmission system, a first modular multilevel converter MMC is avoided1Locked due to fault, and then flows through the first modular multilevel flexible direct current converter MMC1Must be within a defined range for safe operation. Direct Current (DCB) set to flow through first direct current breaker1And a second DC breaker DCB2All fault currents of are ibAnd a first direct current breaker DCB is arranged under the condition that a double-end modular multilevel flexible direct current transmission system generates an inter-electrode fault of a direct current line1And a second DC breaker DCB2Before action, a first modular multi-level flexible direct current converter MMC1Bridge arm current allowable limit value of iaAt t, in the event of a fault between the DC lines in step (8)1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1And (9) calculating the alternating current i of the double-end modularized multi-level flexible direct current transmission systempAnd the step (10) of flowing through a first modular multilevel flexible direct current converter MMC1Fault current i of bridge arm2Obtaining a first modular multilevel converter MMC in the case of interpolar fault of the direct-current line1The boundary conditions before occlusion are:
Figure BDA0002074644060000091
(12) locking a front limit fixed condition by using the first modular multilevel flexible direct current converter MMC1 in the step (11), according to the equivalent capacitor C ', the equivalent inductor L ', the equivalent resistor R ' in the steps (4) to (6) and the steps (8) to (8)10) At t in the event of inter-pole fault of medium DC line1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1AC current ipAnd a first modularized multi-level flexible direct current converter MMC flows through1Fault current i of bridge arm2And matching the superconducting current limiter at t when the double-end modular multilevel flexible direct current transmission system fails through calculation1In-stage DC line with current limiting reactance LdThe value range of (a);
(13) limiting reactance L according to step (12)dBy the current-limiting reactance LdThe maximum value in the value range is used as the current-limiting reactance of the direct-current line when the double-end modular multilevel flexible direct-current power transmission system has an inter-electrode fault of the direct-current line;
(14) traversing the n segments of the superconducting current limiter impedance linearization in the step (3) and the corresponding first-order impedance function R'sAnd (t) repeating the steps (8) to (13), and calculating to obtain the value range of the current-limiting reactance of the direct-current line when the double-end modular multi-level flexible direct-current power transmission system fails, so that the modular multi-level converter MMC can be effectively protected under the condition of no locking when the double-end modular multi-level flexible direct-current power transmission system has an inter-electrode direct-current line fault.

Claims (1)

1. A method for selecting a current-limiting reactance of a flexible direct-current transmission system based on a superconducting current limiter is characterized by comprising the following steps:
(1) constructing a double-ended modular multilevel flexible DC power transmission system with a superconducting current limiter, the double-ended modular multilevel flexible DC power transmission system comprising a first AC line simulated impedance Z1The second AC line analog impedance Z2First modular multi-level flexible direct current converter MMC1Second modular multi-level flexible direct current converter MMC2First direct current breaker DCB1And a second DC breaker DCB2And a third DC breaker DCB3And a fourth DC breaker DCB4A first current limiting reactor Ld1A second current limiting reactor Ld2A third current limiting reactor Ld3A fourth current limiting reactor Ld4And a superconducting current limiter; the first modular multilevel flexible direct current converter MMC1Inlet end and second modular multi-level flexible direct current converter MMC2Respectively through a first AC line to simulate an impedance Z1And a second AC line analogue impedance Z2To the alternating current network AC1And the alternating current network AC2Connected, a first current limiting reactor Ld1And a second current limiting reactor Ld2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1And a third current limiting reactor Ld3And a fourth current limiting reactor Ld4Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2Inlet end of, a first dc breaker DCB1And a second DC breaker DCB2Are respectively arranged on a first modularized multi-level flexible direct current converter MMC1Of the third direct current breaker DCB2And a fourth DC breaker DCB2Are respectively arranged on a second modularized multi-level flexible direct current converter MMC2The superconducting current limiter SFC L is arranged on a negative direct current circuit of the double-end flexible direct current transmission system and is positioned on a second direct current breaker DCB2And a second DC breaker DCB2To (c) to (d);
(2) by using the following formula, the dynamic impedance R of the superconducting current limiter when the DC line interelectrode fault occurs at the outlet end of the superconducting current limiter SFC L in the double-end flexible DC power transmission system is calculateds(t)：
Figure FDA0002423894000000011
Wherein R ismWhen the superconducting current limiter is connected into a double-end modular multi-level flexible direct current transmission system, the maximum resistance value T of the superconducting current limiter in a quench statesThe state time constant of the superconducting current limiter is converted from a superconducting state to a quench state and is obtained by a nameplate of the superconducting current limiter, t is the operation time of the double-end flexible direct current transmission system, tqFor critical current flowing through superconducting current limiterTime of day;
(3) utilizing a least square fitting method to obtain the dynamic impedance R of the superconducting current limiter in the step (2)s(t) is piecewise linearized into n first-order impedance functions R 'varying with time t's(t)：
R′s(t)＝Ri(ti)(i＝1，2，3，...，n)
(4) MMC (modular multilevel flexible direct current converter) according to first modular multilevel1The number N of submodules and the capacitance C of the submodules0Calculating the equivalent capacitance C' of a fault loop when the double-end modular multilevel flexible direct current transmission system has an inter-pole fault of a direct current line by using the following formula:
Figure FDA0002423894000000021
wherein N is a first modular multi-level flexible direct current converter MMC1Number of sub-modules SM, C0The capacitance size in a single modular multilevel sub-module SM is obtained;
(5) calculating the equivalent inductance L' of a fault loop of the power transmission system when the double-end modular multilevel flexible direct current power transmission system has an inter-pole fault of a direct current line according to the following formula:
Figure FDA0002423894000000022
wherein, LsBridge arm inductor L passing through MMC1 of first modular multilevel converter for short-circuit current1DC line reactance for double-ended modular multilevel flexible DC transmission system, LdA current-limiting reactance of a direct-current line of a double-end modular multilevel flexible direct-current transmission system to be solved;
(6) the fault loop equivalent resistance R' is calculated using the following equation:
Figure FDA0002423894000000023
wherein R isarmFor dual-end modular multi-level flexibleBridge arm resistance R of first modular multilevel flexible direct current converter MMC1 in linear direct current transmission system1Is a direct current line resistance R in a double-end modular multi-level flexible direct current transmission systemfFor fault resistance when a double-ended modular multilevel flexible DC power transmission system has an interpolar fault in the DC lines, R1(t1) Impedance of the superconducting current limiter in a quench state when the double-end modular multilevel flexible direct current transmission system fails;
(7) according to the equivalent capacitance C ' of the step (4), the equivalent inductance L ' of the fault loop of the step (5), the equivalent resistance R ' of the fault loop of the step (6) and the equivalent capacitance voltage v of the fault loopcAnd obtaining a stage mathematical expression when the double-end modular multilevel flexible direct-current transmission system has an interpolar fault of a direct-current line, wherein the stage mathematical expression is as follows:
Figure FDA0002423894000000024
(8) according to the stage mathematical expression in the step (7), calculating the time t when the inter-electrode fault of the direct current line occurs in the double-end modular multi-level flexible direct current transmission system by using the following formula1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1：
Figure FDA0002423894000000025
Where σ is the damping coefficient, σ ═ R '/2L ', ω ' is the oscillation angular frequency,
Figure FDA0002423894000000026
(9) setting the transmission power balance of an alternating current power grid and a modular multilevel converter when the double-end modular multilevel flexible direct current transmission system has interpolar faults of a direct current line, and obtaining the alternating current i of the double-end modular multilevel flexible direct current transmission system according to the following formulap：
Figure FDA0002423894000000031
Wherein, VdcDC voltage, I, for a double-ended modular multilevel flexible DC transmission systemdDirect current, V, for a double-ended modular multilevel flexible direct current transmission systemlThe voltage of an alternating current network line of the double-end modular multi-level flexible direct current transmission system;
(10) according to step (8) fault current i1And the alternating current i of step (9)pAnd calculating to obtain a first modular multilevel flexible direct current converter MMC in the double-end modular multilevel flexible direct current transmission system by using the following formula1At t at superconducting current limiter impedance1MMC (modular multilevel flexible direct current converter) capable of flowing through first modular multilevel in stage1Fault current i of bridge arm2：
Figure FDA0002423894000000032
(11) When a double-end modular multi-level flexible direct current transmission system has an inter-pole fault of a direct current line, a first direct current breaker DCB is arranged to flow through1And a second DC breaker DCB2All fault currents of are ibAnd a first direct current breaker DCB is arranged under the condition that a double-end modular multilevel flexible direct current transmission system generates an inter-electrode fault of a direct current line1And a second DC breaker DCB2Before action, a first modular multi-level flexible direct current converter MMC1Bridge arm current allowable limit value of iaAt t, in the event of a fault between the DC lines in step (8)1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1And (9) calculating the alternating current i of the double-end modularized multi-level flexible direct current transmission systempAnd the step (10) of flowing through a first modular multilevel flexible direct current converter MMC1Fault current i of bridge arm2Obtaining a first modular multilevel converter MMC in the case of interpolar fault of the direct-current line1The boundary conditions before occlusion are:
Figure FDA0002423894000000033
(12) locking a front limit fixed condition by using the first modular multilevel flexible direct current converter MMC1 in the step (11), and according to the equivalent capacitor C ', the equivalent inductor L ', the equivalent resistor R ' in the step (4) to the step (6) and the inter-pole fault of the direct current line in the step (8) to the step (10) at t1Staged flow through first DC breaker DCB1And a second DC breaker DCB2Fault current i of1AC current ipAnd a first modularized multi-level flexible direct current converter MMC flows through1Fault current i of bridge arm2And matching the superconducting current limiter at t when the double-end modular multilevel flexible direct current transmission system fails through calculation1In-stage DC line with current limiting reactance LdThe value range of (a);
(13) limiting reactance L according to step (12)dBy the current-limiting reactance LdThe maximum value in the value range is used as the current-limiting reactance of the direct-current line when the double-end modular multilevel flexible direct-current power transmission system has an inter-electrode fault of the direct-current line;
(14) traversing the n segments of the superconducting current limiter impedance linearization in the step (3) and the corresponding first-order impedance function R'sAnd (t) repeating the steps (8) to (13), and calculating to obtain the value range of the current-limiting reactance of the direct-current line when the double-end modular multi-level flexible direct-current power transmission system fails, so that the modular multi-level converter MMC can be effectively protected under the condition of no locking when the double-end modular multi-level flexible direct-current power transmission system has an inter-electrode direct-current line fault.
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