CN115425623A - Transient current ratio-based multi-terminal flexible direct-current transmission line pilot protection method - Google Patents

Transient current ratio-based multi-terminal flexible direct-current transmission line pilot protection method Download PDF

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CN115425623A
CN115425623A CN202210932453.2A CN202210932453A CN115425623A CN 115425623 A CN115425623 A CN 115425623A CN 202210932453 A CN202210932453 A CN 202210932453A CN 115425623 A CN115425623 A CN 115425623A
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fault
protection
current
voltage
line
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李永丽
刘佳硕
陈晓龙
刘宁宁
周宁
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Tianjin University
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Tianjin University
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    • 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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H1/00—Details of emergency protective circuit arrangements
    • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
    • 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
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • 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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  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Testing Of Short-Circuits, Discontinuities, Leakage, Or Incorrect Line Connections (AREA)

Abstract

The invention relates to a transient current ratio-based multi-terminal flexible direct-current transmission line pilot protection method, which comprises the following steps of S1: determining the internal and external fault identification criteria; s2: determining a fault starting criterion; s3: determining fault pole selection criteria; s4: a protection scheme is determined. And continuously collecting current and voltage data at two sides of the line, and judging that the direct current transmission line is likely to have a fault if the voltage at the protection installation position of the direct current line at a certain end meets the protection starting criterion, and starting the protection starting element at the end. And the transient current at the protection installation position and the transient current at the direct current side outlet of the MMC at the side are collected in a protection mode, the ratio is calculated, faults inside and outside the area are judged, the transient voltage of the positive electrode line and the negative electrode line is collected, the energy ratio of the transient voltage is calculated, and the fault type is judged to select the fault pole of the fault type. The invention can quickly and reliably identify the internal and external faults of the direct current circuit area, reliably protect the whole length of the circuit, has strong transition resistance, does not need real-time synchronization of data at two ends of the circuit, and has higher action speed than the traditional current differential protection.

Description

Transient current ratio-based multi-terminal flexible direct-current transmission line pilot protection method
Technical Field
The invention belongs to the technical field of protection and control of power systems, and particularly relates to a pilot protection method for a multi-terminal flexible direct-current transmission line based on a transient current ratio.
Background
The flexible direct-current transmission technology based on the Modular Multilevel Converter (MMC) can improve the grid-connected efficiency of new energy power generation, and has the advantages of quick, independent and controllable active power and reactive power, no commutation failure, high waveform quality and the like, so the flexible direct-current transmission technology becomes the current popular research. In order to fully utilize renewable energy resources in Zhangjiakou areas, the national power grid builds a Zhang Beijing four-end flexible direct current power grid demonstration project based on MMC in Beijing and Hebei, and the appearance of a multi-port flexible direct current power grid (MTDC) brings new challenges to the existing direct current line protection technology. Compared with a conventional direct-current transmission system, the flexible direct-current transmission system has smaller damping, and after a direct-current line fault occurs, a power grid generates a large fault component instantly, so that the capacitor of the converter discharges rapidly and exceeds the bearing capacity of power electronic devices of the converter station within a few milliseconds, and the flexible direct-current power grid requires shorter protection action time.
The existing high-voltage direct-current line protection is divided into double-end quantity protection and single-end quantity protection. At present, the research focus of double-end line protection is mainly based on the traveling wave principle and mutation analysis.
In the prior art, the internal and external faults of the area are identified by using the ratio of the transient voltage energy at two ends of a direct current reactor, the method has good transition resistance, high-frequency components are extracted, the calculation is complex, and the requirement on sampling equipment is high; the literature also describes a pilot protection scheme based on the wave impedance and the phase, but the two schemes require higher sampling frequency and lack of applicability; the document also describes a direct current power grid fault identification scheme based on the traveling wave principle, and fault discrimination is performed according to the time difference of the fault traveling wave reaching each converter station, so that higher sampling frequency is also required, and fast communication and information synchronization between the stations are required.
Smoothing reactors are arranged at two ends of a direct current transmission line, so that the fault component after the fault is limited to rapidly rise, and the direct current reactors are used as boundary elements for numerous single-end quantity protection. The document proposes a single-end protection scheme by taking the voltage of a direct-current reactor as a protection criterion; the document proposes that a direct current reactor has a good smoothing effect on voltage fault traveling waves of external faults, and a fault line is selected according to the voltage change rate of a direct current line; in the literature, based on the characteristic that a reactor presents higher impedance to a high-frequency component, the energy of the high-frequency component of current is extracted by utilizing wavelet transformation to serve as a protection action basis, but data processing is complex. However, the single-end protection method is difficult to ensure the full length of the protection circuit, and has the problems that high-resistance faults cannot be identified and the like.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a multi-end flexible direct-current transmission line pilot protection method based on a transient current ratio, after a direct-current line fault occurs in a multi-end flexible direct-current power grid, an MMC can quickly generate fault current and flow into a fault point along the direct-current line, and due to the existence of a direct-current reactor, the impedance of the fault current of the MMC flowing from the fault line to the fault point is far smaller than the impedance of other paths, so most of MMC fault current directly flows into the fault point through the fault line; by matching with the traveling wave process analysis after the fault, the transient characteristic difference of the inner fault and the outer fault is obtained theoretically, the transient current ratio of two ends of the fault line is larger than 0.5, the transient current ratio of one end of the non-fault line is smaller than 0.5, and pilot protection can be formed according to the characteristic; the protection scheme has small calculated amount, can quickly identify the internal and external faults of the area, reliably protects the whole length of the line and has strong transition resistance.
The technical problem to be solved by the invention is realized by the following technical scheme:
a transient current ratio-based pilot protection method for a multi-terminal flexible direct-current transmission line is characterized by comprising the following steps: the method comprises the following steps:
s1, determining internal and external fault identification criteria of a zone;
definition K mn To protect P mn Installation transient current i mn And the outlet transient current i of the local side MMC direct current side MMCm Ratio of (i) to (ii)
Figure BDA0003782185120000021
At the beginning of a fault, K at both ends of the faulty line mn >0.5 that the non-faulted line must have one end K mn <0.5, forming pilot protection according to the characteristics, and setting K obtained by protection calculation at two ends of a fault line mn Are each K a And K b The protection action condition is the length T of the data window after the protection is started W2 Internal and always satisfy
Figure BDA0003782185120000022
When T is W2 When the value is small, the number of sampling points is small, and the reliability of protection is low;
when T is W2 When the value is larger, the time of protection action is longer, the protection principle is more influenced by refraction, reflection and the like of fault traveling waves among lines, and the protection reliability is reduced; the protection quickness and reliability are comprehensively considered, and the length T of a data fetching window W2 ＝0.5ms；
S2, establishing a protection starting criterion;
when the system normally operates, the voltage of a direct current line fluctuates around a rated voltage; when short-circuit fault occurs, the line voltage is obviously reduced, a low-voltage starting scheme is adopted, the direct-current voltage to ground at the installation position is detected, and the time window length T is long W1 Inner sampled voltage amplitude | U t | is lower than a set value U set When the protection is started, the protection starting set value is taken as U set ＝0.8U N Wherein U is N For the rated DC voltage, the time window length T is taken W1 =0.3ms, is protectedThe starting criterion is as follows:
|U t |＜0.8U N
s3, establishing a fault pole selection criterion;
after the single-pole ground fault occurs, the transient voltage component amplitude of the non-fault pole generated by the coupling action is smaller than that of the fault pole; after a bipolar short-circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC current converter, the voltage transient component amplitudes of the two poles are basically the same, and transient voltage energy E is defined mn For protecting the length T of the time window after starting W3 Inner protection P mn Integration of the square of the voltage transient u at the installation, i.e.
Figure BDA0003782185120000031
Definition of beta mn To protect P mn Ratio of transient voltage energies of positive and negative lines at the installation, i.e.
Figure BDA0003782185120000032
In the formula, E mnp And E mnn Are respectively protection P mn The transient voltage energy of the positive and negative direct current lines at the installation position;
to increase the reliability of fault pole selection, T is taken W3 =0.5ms, in beta mn Is the basis of pole selection, i.e.
Figure BDA0003782185120000033
S4, determining a protection scheme;
when the protection detects that the amplitude of the line voltage is lower than 0.8U N Judging that the direct current transmission line is likely to have a fault, and starting a protection process; transient current i at protective acquisition protective installation position mn Transient current i at the outlet of the MMC direct current side of the local side MMCm And calculate K mn Collecting transient voltages of positive and negative lines and calculatingTransient voltage to energy ratio beta mn ；
When K is mn >When 0.5 hour, the protection sends tripping permitting signals to the local terminal and the opposite terminal; when the protection receives a tripping-allowed signal sent by two ends of a line, judging the fault in the area;
when beta is mn >2, judging the fault to be a positive pole fault;
when beta is mn <When 0.5 hour, the cathode fault is judged;
when beta is more than or equal to 0.5 mn Judging the bipolar fault when the current is less than or equal to 2; and after the fault line is selected and the fault type is judged, the fault pole DCCB of the fault line carries out switching-off operation.
The invention has the advantages and beneficial effects that:
1. the invention has small calculated amount, can reliably and quickly judge the faults inside and outside the direct current line area and protect the total length of the line.
2. The direct current line protection provided by the invention has strong transition resistance and noise resistance, is not influenced by line distributed capacitance, and can adapt to the change of operation modes.
3. The invention does not need data synchronization at two ends of the line, and has lower requirement on the reliability of a communication channel.
Drawings
FIG. 1 is a diagram of a four-terminal flexible DC power grid topology;
FIG. 2 is a fault current flow path diagram of an MMC;
FIG. 3 is a fault additional network diagram of a four-terminal flexible direct-current power grid;
FIG. 4 is a flow chart of the present invention;
FIG. 5 shows a line L line23 A midpoint anode metallic grounding simulation experiment chart;
FIG. 6 shows a line L line23 A simulation experiment chart of midpoint anode high-resistance grounding.
Detailed Description
The present invention is further illustrated by the following specific examples, which are intended to be illustrative, not limiting and are not intended to limit the scope of the invention.
The invention provides a transient current ratio-based pilot protection method for a multi-terminal flexible direct-current transmission line, which is characterized in that a recognition criterion is formed by utilizing the transient current ratio of a protection installation position after a fault is detected and the direct-current outlet of an MMC (modular multilevel converter) at the side, and when the fault occurs in a region, the transient current ratios of two ends of the line are both greater than 0.5; when the fault is out of the area, the transient current ratio of at least one end is less than 0.5; the protection starting time window of the protection scheme is 0.3ms, fault positioning and fault pole selection are carried out simultaneously, and the time window is 0.5ms; the fault identification is completed within 3ms after the fault occurs.
The four-end flexible direct current power grid structure is shown in fig. 1, four converters of the four-end flexible direct current power grid structure all adopt a symmetrical bipolar MMC structure, wherein an MMC sub-module is a half-bridge sub-module, and two ends of a direct current transmission line are provided with a direct current circuit breaker DCCB and a direct current reactor. L is a radical of an alcohol linemn For dc transmission lines, P mn Protective devices, BUS, mounted for the ends and heads of the lines m Is a dc bus (where m, n =1,2,3, 4).
The half-bridge MMC does not have the direct-current fault self-clearing capacity, when a direct-current line has a short-circuit fault, the line voltage rapidly drops and is lower than the total voltage of the sub-module capacitors input by each phase of bridge arm, so that the sub-module capacitors are rapidly discharged to a fault point, and the fault current flowing through the MMC rapidly rises. Before the MMC converter station is locked, fault transient current is mainly the discharge current of a sub-module capacitor, and simultaneously contains a small amount of short-circuit current fed into an alternating current system. Taking the positive earth fault of the dc line as an example, the fault current path of the fault pole MMC is shown in fig. 2, wherein the solid line current path is the sub-module capacitor discharge path, the dotted line current path is the ac system feed-in short circuit current path, BUS is the dc BUS, Z eq Is the equivalent impedance from the positive outlet of the MMC to a fault point, L arm Bridge arm inductance, and sub-module capacitance. After a fault occurs, the sub-module capacitor discharge current flows to a direct current bus from the current converter, short-circuit current fed into the alternating current system flows alternately among bridge arms along with the periodic change of alternating-current side voltage, and the flowing direction of the short-circuit current is the same as that of the sub-module capacitor discharge current. Transient current detected at an outlet of a direct current side of the MMC is superposition of sub-module capacitor discharge current and short-circuit current fed in by an alternating current system, the direction of the transient current is always from the MMC to a direct current bus, and alternating current is not distinguishedThe current system feeds a short circuit current and a sub-module capacitor discharge current, both of which are collectively referred to as MMC fault current.
By a line L line23 Taking the case that a metallic ground fault occurs at the positive electrode F point as an example, a fault output model of the MMC current converter is equivalent to a time-varying current source, the magnitude of the time-varying current source is the same as the fault current flowing through the MMC, the voltage of the fault point is suddenly changed to 0, equivalently, a step fault voltage source is introduced into the point, and the magnitude of the fault voltage source is the opposite value of the operation voltage at the moment before the fault of the point. According to the superposition theorem, a fault additional network of a four-terminal flexible direct-current power grid can be obtained as shown in fig. 3, wherein i MMCm Is MMC m A direct current side outlet fault current; i all right angle mn To protect P mn Transient current of the line at the installation site; -E is a fault voltage source, E is the operating voltage before fault at the fault point; z L Is a direct current reactor impedance; z 12 、Z 41 、Z 34 、Z 2F And Z 3F Is the line impedance. The specified current flows from the bus to the line to be the positive direction of the line current, and flows from the MMC to the bus to be the positive direction of the outlet current at the direct current side of the MMC. Definition K mn For protecting the installation transient current i mn With the outlet transient current i of the local side MMC DC side MMCm Ratio of (i) to (ii)
Figure BDA0003782185120000051
The fault current and node voltage on the line in fig. 3 can be regarded as the superposition of respective independent actions of an MMC equivalent current source and a fault voltage source at a fault point, and the fault voltage traveling wave generated under the independent action of the fault voltage source is counted as traveling wave W u Sum traveling wave V u 。
Travelling wave W u To the BUS 2 Then, in the voltage traveling wave W u BUS under action 2 Voltage drop, MMC 2 Generation of a fault current, denoted i MMC2 。i MMC2 The incoming fault point has two paths, shown by the blue dotted line in FIG. 3, which are BUS respectively 2 -F and BUS 2 -BUS 1 -BUS 4 -BUS 3 F, electricity to be flowed into both pathsFlow component is measured i 23MMC2 And i 21MMC2 Impedance Z on the first path B1 ＝Z L +Z 2F Impedance Z of the second path B2 ＝7Z L +Z 12 +Z 41 +Z 34 +Z 3F . The line and DC reactor through which the second path flows have a greater impedance, obviously Z B1 >Z B2 Thus, can obtain
Figure BDA0003782185120000061
Fault travelling wave W u Backward BUS after refraction 1 While propagating, line L line12 The voltage traveling wave amplitude is a negative value, so that the current traveling wave amplitude is a negative value. Putting the fault voltage source in protection P 21 The component of the current fault caused by the installation is counted as i 21W Knowing that i 21W <0. According to the superposition theorem, P is protected 21 The detected transient current is formed by fault current traveling wave and MMC 2 Fault currents being formed jointly, i.e. i 21 ＝i 21MMC2 +i 21W From kirchhoff's law, i can be derived 23 ＝i MMC2 -i 21 ＝i 23MMC2 -i 21W . Due to i 21W <0, can yield K 23 And K 21 Satisfies the following inequality:
Figure BDA0003782185120000062
by the same token can obtain
Figure BDA0003782185120000063
Will line L line41 The sum of the transient currents of the capacitors to the ground is recorded as sigma i C The forward direction of the current flowing from the line to the ground is defined, and according to kirchhoff's current law, Σ i can be obtained C ＝i 14 +i 41 . When the fault voltage traveling wave reaches the line L line41 In the time, because the voltage traveling wave is a negative value, the line voltage begins to drop, and in the process of dropping the line voltage, the earth capacitance is discharged, and the sum sigma i of the transient currents of the earth capacitance C Less than 0, so that i 14 +i 41 <0, at this time, there is i 14 Or i 41 Less than 0, protection P 14 And protection P 41 Must have one end measuring K mn <0.5。
It can be concluded that after a fault occurs in the dc line, two paths of fault current generated by the MMC flow into the fault point, and for the MMC at both ends of the fault line, the impedance of the two paths of the fault current has a large difference, so that the component of the fault current flowing into the two paths has a large difference, and most of the fault current directly flows into the fault point through the fault line. At the beginning of a fault, K at both ends of the faulty line mn >0.5 that the non-faulted line must have a terminal K mn <0.5, pilot protection can be constructed according to the characteristics.
A transient current ratio-based pilot protection method for a multi-terminal flexible direct-current transmission line comprises the following steps:
1. establishing a protection starting criterion:
when the system normally operates, the voltage of a direct current line fluctuates around a rated voltage; when a short-circuit fault occurs, the line voltage is significantly reduced. The method adopts a low-voltage starting scheme, detects the direct-current voltage to ground at the protection installation position, and has the time window length T W1 Inner sampled voltage amplitude | U t I is lower than the set value U set When the protection is started, the protection is started. The low voltage setting value needs to avoid the voltage during normal operation, and the set value of protection starting is U in consideration that the protection starting cannot be too frequent and the protection starting can still be reliably started when the fault of the transient resistor occurs set ＝0.8U N Wherein U is N For the rated DC voltage, the time window length T is taken W1 =0.3ms. The obtained protection start criterion is as follows:
|U t |＜0.8U N (5)
2. establishing a fault positioning criterion:
after the protection is started, measuring transient current i at the protection installation position mn With the MMC direct current side of the local sideMouth transient current i MMCm And calculate K mn . At the beginning of a fault, K at both ends of the faulty line mn >0.5 that the non-faulted line must have a terminal K mn <0.5, pilot protection can be constructed according to the characteristics. Setting K obtained by protection calculation at two ends of line mn Are each K a And K b The protection action condition is the length T of the data window after the protection is started W2 In all the time satisfy
Figure BDA0003782185120000071
When T is W2 When the value is small, the number of sampling points is small, and the reliability of protection is low; when T is W2 When the value is larger, the time of protection action is longer, the influence of refraction, reflection and the like of fault traveling waves among lines on the protection principle is larger, and the protection reliability is reduced on the contrary. The protection quickness and reliability are comprehensively considered, and the length T of a data fetching window W2 ＝0.5ms。
3. Establishing fault pole selection criterion:
after a single-pole ground fault occurs, the transient voltage component amplitude of a non-fault pole generated by the coupling action is smaller than that of a fault pole; after a bipolar short-circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC current converter, the voltage transient component amplitudes of the two poles are basically the same. Defining transient voltage energy E mn For protecting the length T of the time window after starting W3 Internal protection P mn Integration of the square of the voltage transient u at the installation, i.e.
Figure BDA0003782185120000072
Definition of beta mn To protect P mn Ratio of transient voltage energy of positive and negative lines at the installation, i.e.
Figure BDA0003782185120000081
In the formula, E mnp And E mnn Are respectively protection P mn And the transient voltage energy of the positive and negative direct current lines is arranged at the installation position. To increase the reliability of fault pole selection, take T W3 =0.5ms, in beta mn Is the basis of pole selection, i.e.
Figure BDA0003782185120000082
4. Protection process:
the protection method of the present invention has a flow as shown in fig. 4, when the protection detects that the line voltage amplitude is lower than 0.8U N And judging that the direct current transmission line is likely to have a fault, and starting a protection process. Transient current i at protective acquisition protective installation mn Transient current i at the outlet of the MMC direct current side of the local side MMCm And calculate K mn Collecting transient voltage of positive and negative electrode lines and calculating transient voltage energy ratio beta mn . When K is mn >And 0.5, the protection sends tripping-allowed signals to the local terminal and the opposite terminal, and when the protection receives the tripping-allowed signals sent by the two ends of the line, the fault in the area is judged.
When beta is mn >2, judging the fault to be a positive pole fault;
when beta is mn <When 0.5 hour, the cathode fault is judged;
when beta is more than or equal to 0.5 mn When the current is less than or equal to 2, the bipolar fault is judged. And after the fault line is selected and the fault type is judged, the fault pole DCCB of the fault line carries out switching-off operation.
Simulation verification
A Zhangbei four-end flexible direct current network model is built in a PSCAD/EMTDC simulation platform for simulation verification, the topological structure of the model is shown in figure 1, wherein MMC1-MMC4 converter stations are Fengning converter stations, beijing converter stations, zhangbei converter stations and health converter stations respectively. The Zhang Beijing, beijing and Kangbao stations adopt fixed active power control and fixed reactive power control, the Fengning station adopts fixed reactive power control and fixed direct current voltage control, and the rated direct current voltage of a direct current power grid is +/-500 KV. The overhead line between each converter station adopts a frequency-dependent line model with the positive pole and the negative pole erected on the same tower, the length of the frequency-dependent line model is shown in figure 1, and the sampling frequency is 20kHz.
Metallic ground fault simulation
DC line L line23 The simulation result of the neutral-point metallic positive ground fault is shown in fig. 5. Fig. 5a shows the positive and negative voltages at two ends of the fault line, 0.35ms after the fault occurs, the fault traveling wave is transmitted to the protection installation positions at two ends of the line, the positive voltage rapidly drops and reaches the protection starting set value, and the protection starting is carried out after 0.3ms. The voltage of the negative electrode circuit fluctuates slightly due to the coupling effect, the voltage variation is far smaller than that of the positive electrode circuit, and the transient voltage energy ratio beta of the two-electrode circuit is calculated after the protection is started mn Obtaining beta 23 ＝33，β 32 =69, it can be determined that the positive electrode has failed. Although the voltage of the negative pole line is close to the protection starting threshold value, the negative pole protection can be reliably locked without misoperation due to the existence of the fault pole selection element.
Calculating transient current i of protection installation position on positive line mn Transient current i at outlet of local side MMC MMCm Ratio K of mn Protection of two ends of the same line to measured K mn Drawing in the same coordinate system to obtain waveforms as shown in FIG. 5b, with the time corresponding to the dotted abscissa line being the data window T W2 K before protection is no longer enabled mn Analysis was performed, which was plotted as 0. Analyzing fig. 5b, it can be seen that the protection at both ends of the faulty line senses the faulty component first and starts, so that the protection P 23 And P 32 First-in data window T W2 Sampling and calculating K mn (ii) a The further the protection is from the fault point, the later the protection is initiated. Within 2.5ms after the fault occurs, K is arranged at two ends of the fault line mn Are all larger than 0.5, and a non-fault line must have one end K mn <0.5。
High resistance ground fault simulation
In the theoretical analysis of the metallic fault, the MMC fault current and the fault current traveling wave generated by the fault voltage source are simultaneously considered, and theoretically, the introduction of the transition resistance only changes the transient current flowing through the line and does not change the distribution characteristics of the transient current. The method uses the transient current ratio after the fault as the action criterion, and the transient current is measuredThe influence of the change is small, so the protection action criterion is still suitable for high-resistance faults in theory. Provided with a line L line23 The positive pole earth fault occurs at the midpoint, the transition resistance is 300 omega, and the obtained K mn The waveform is shown in fig. 6. After addition of the transition resistance, K 23 And K 32 The waveform is similar to that of metallic fault, the protection at two ends of the fault line can reliably act, the protection of the non-fault line can not malfunction, and the influence of the transition resistance on the protection principle is small.
By analyzing the distribution characteristics of transient current on the line when the direct-current line fails, the fault locating method based on the ratio of the transient current of the direct-current line to the fault current of the MMC realizes fault locating, is matched with fault starting criteria and pole selection criteria, considers the fault traveling wave transmission speed and the inter-station communication time, and can quickly locate the fault pole of the fault line within 3ms after the direct-current line fails. The protection scheme only needs to transmit an allowable trip signal, does not need data synchronization, has low requirement on the reliability of communication, has stronger anti-transition resistance capability and anti-noise capability, and is not influenced by line distributed capacitance.
Although the embodiments of the present invention and the accompanying drawings are disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and appended claims, and therefore, the scope of the invention is not limited to the disclosure of the embodiments and drawings.

Claims (1)

1. A transient current ratio-based pilot protection method for a multi-terminal flexible direct-current transmission line is characterized by comprising the following steps: the method comprises the following steps:
s1, determining internal and external fault identification criteria of a zone;
definition K mn To protect P mn Installation transient current i mn With the outlet transient current i of the local side MMC DC side MMCm Ratio of (i) to (ii)
Figure FDA0003782185110000011
At the beginning of a fault, K at both ends of the faulty line mn >0.5 that the non-faulted line must have one end K mn <0.5, forming pilot protection according to the characteristics, and setting K obtained by protection calculation at two ends of a fault line mn Are each K a And K b The protection action condition is the length T of the data window after the protection is started W2 In all the time satisfy
Figure FDA0003782185110000012
When T is W2 When the value is small, the number of sampling points is small, and the reliability of protection is low;
when T is W2 When the value is larger, the protection action takes longer time, the protection principle is more greatly influenced by refraction, reflection and the like of fault traveling waves among lines, and the protection reliability is reduced on the contrary; the protection rapidity and reliability are comprehensively considered, and the length T of the data fetching window W2 ＝0.5ms；
S2, establishing a protection starting criterion;
when the system normally operates, the voltage of a direct current line fluctuates around a rated voltage; when short-circuit fault occurs, the line voltage is obviously reduced, a low-voltage starting scheme is adopted, the direct-current voltage to ground at the protection installation position is detected, and the time window is long T W1 Inner sampled voltage amplitude | U t | is lower than a set value U set When the protection is started, the protection starting set value is taken as U set ＝0.8U N Wherein U is N Taking the time window length T for the rated DC voltage W1 =0.3ms, and the obtained protection starting criterion is as follows:
|U t |＜0.8U N
and step 3: establishing fault pole selection criterion;
after the single-pole ground fault occurs, the transient voltage component amplitude of the non-fault pole generated by the coupling action is smaller than that of the fault pole; after a bipolar short-circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC current converter, the voltage transient component amplitudes of the two poles are basically the same, and transient voltage energy is definedE mn For protecting the length T of the time window after starting W3 Internal protection P mn Integration of the square of the voltage transient u at the installation, i.e.
Figure FDA0003782185110000021
Definition of beta mn To protect P mn Ratio of transient voltage energy of positive and negative lines at the installation, i.e.
Figure FDA0003782185110000022
In the formula, E mnp And E mnn Are respectively protection P mn Transient voltage energy of positive and negative direct current lines at the installation site;
to increase the reliability of fault pole selection, take T W3 =0.5ms, in beta mn Is the basis of pole selection, i.e.
Figure FDA0003782185110000023
And 4, step 4: determining a protection scheme;
when the protection detects that the amplitude of the line voltage is lower than 0.8U N Judging that the direct current transmission line is likely to have a fault, and starting a protection process; transient current i at protective acquisition protective installation mn Transient current i at the outlet of the MMC direct current side of the local side MMCm And calculate K mn Collecting transient voltage of positive and negative electrode lines and calculating transient voltage energy ratio beta mn ；
When K is mn >When 0.5 hour, the protection sends tripping permitting signals to the local terminal and the opposite terminal; when the protection receives a tripping-allowed signal sent by two ends of a line, judging the fault in the area;
when beta is mn >2, judging the positive pole fault;
when beta is mn <When 0.5 hour, the cathode fault is judged;
when beta mn is more than or equal to 0.5 and less than or equal to 2, judging the bipolar fault; and after the fault line is selected and the fault type is judged, the fault pole DCCB of the fault line carries out switching-off operation.
CN202210932453.2A 2022-08-04 2022-08-04 Transient current ratio-based multi-terminal flexible direct-current transmission line pilot protection method Pending CN115425623A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116979528A (en) * 2023-09-22 2023-10-31 国网湖北省电力有限公司 Method, device and medium for quickly starting low-voltage ride through of power electronic converter

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116979528A (en) * 2023-09-22 2023-10-31 国网湖北省电力有限公司 Method, device and medium for quickly starting low-voltage ride through of power electronic converter
CN116979528B (en) * 2023-09-22 2023-12-15 国网湖北省电力有限公司 Method, device and medium for quickly starting low-voltage ride through of power electronic converter

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