CN109638796B - Differential protection method for long-distance high-voltage direct-current transmission line - Google Patents
Differential protection method for long-distance high-voltage direct-current transmission line Download PDFInfo
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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
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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
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- 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 relates to a differential protection method for a long-distance high-voltage direct-current transmission line, which comprises the following steps of: (1) decoupling voltage and current sampling values at two ends of the line, which are acquired by a voltage transformer and a current transformer at the outlets of the converter stations at two sides of the line, to obtain voltage and current line mode components and ground mode components at two sides of the line; (2) obtaining a line mode difference current and a ground mode difference current of a reference point according to the Bergeron equivalent model; (3) constructing an action criterion of current differential protection of the direct-current transmission line by using the line mode difference current of the reference point; (4) and constructing a fault pole selection criterion by using the ratio of the earth mode difference current and the line mode difference current at the reference point.
Description
Technical Field
The invention relates to the field of protection and control of power systems, in particular to a current differential protection method suitable for a long-distance high-voltage direct-current transmission line.
Background
With the rapid development of power electronic technology, the direct current transmission technology embodies a plurality of advantages which are not possessed by alternating current transmission in long-distance and high-power transmission, the direct current transmission technology develops rapidly, and the domestic direct current transmission project also increases gradually. The direct-current transmission line is the part with the highest fault rate in the whole direct-current transmission project, so the relay protection technology of the direct-current transmission line is of great importance. At present, current differential protection is often used as backup protection of a power transmission line in actual direct-current power transmission engineering, and the influence of factors such as external fault unbalanced current and the like needs to be avoided through a long time delay, so that the selectivity of the current differential protection is ensured.
In order to improve the performance of the current differential protection of the direct-current transmission line, the differential current at a certain reference point of the line can be calculated by utilizing a distributed parameter model to eliminate the influence of the distributed capacitance current during the transient state of the line fault, so that the action performance of the current differential protection is improved. Meanwhile, the transmission line parameters not only have distribution characteristics but also have frequency conversion characteristics, so that the travelling wave dispersion phenomenon on the long-distance direct current transmission line is obvious, and the phenomenon limits the further improvement of the current differential protection performance of the direct current transmission line. Therefore, the speed, the sensitivity and the reliability of the direct-current transmission line differential protection method are obviously improved by designing the direct-current transmission line differential protection method which gives consideration to both the line parameter distribution characteristic and the frequency conversion characteristic.
Disclosure of Invention
The invention provides a direct current transmission line current differential protection method capable of quickly and reliably acting, aiming at the problem that the current differential protection action characteristic of the current direct current transmission line is poor. The technical scheme is as follows:
a differential protection method for a long-distance high-voltage direct-current transmission line is provided, wherein two ends of the transmission line are a J side and a K side, and a midpoint r of the line is taken as a reference point, and comprises the following steps:
(1) voltage transformer at outlets of converter stations on two sides of the line and voltage and current sampling values u at two ends of the line collected by the current transformerJP、uJN、iJP、iJN、uKP、uKN、iKP、iKNDecoupling to obtain voltage and current line mode components u on two sides of the lineJ1、uK1、iJ1、iK1And the earth-mode component uJ0、uK0、iJ0、iK0；
(2) According to the Bergeron equivalent model, the voltage and current line mode component u of the line J side is utilizedJ1、iJ1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesJr1Using the voltage-current line modulus component u on the line K sideK1、iK1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesKr1Line mode differential current i of reference point1diffAs the difference between them:
i1diff＝isjr1-iskr1
similarly, the earth mode difference current i of the reference point can be calculated0diff；
(3) Constructing an action criterion of the current differential protection of the direct-current transmission line by using the line mode difference current of the reference point:
in the formula isetThreshold value, max { i, for current differential protection criterion1unbalanceDenotes the maximum unbalance current value, k, of the differential current of the Bergeron mode under various out-of-region fault conditionsrelIs a reliability factor;
(4) and constructing a fault pole selection criterion by using the ratio of the earth mode difference current and the line mode difference current at the reference point:
in the formula, m is a threshold value, and sufficient sensitivity can be ensured for judging various fault types when a certain margin is considered to be set as 0.1.
Preferably, max { i }1unbalanceAnd the value is the unbalanced current of the differential protection when the metallic fault occurs between the near-end bipolar electrodes outside the protection area. k is a radical ofrelTaking 1.3-1.5.
Drawings
Fig. 1 is a schematic diagram of a high voltage direct current transmission line.
Fig. 2 is a graph showing the variation trend of the linear mode wave velocity and the ground mode wave velocity.
Fig. 3 is a schematic diagram of various types of faults, in which (a) is an inter-pole short-circuit fault of a bipolar line, (b) is a positive-pole line ground fault, and (c) is a negative-pole line ground fault.
Detailed Description
The Bergeron equivalent model of the power transmission line is a constant coefficient distribution parameter model and can better reflect the distribution characteristics of line parameters. The difference current at a certain reference point of the line can be calculated through voltage and current measurement values at two ends of the line by using a Bergeron equivalent model, and the current differential protection criterion is constructed by using the difference current, so that the action characteristic of differential protection can be improved. However, the direct-current transmission line is often long, the traveling wave dispersion phenomenon caused by the frequency-varying characteristic of the direct-current transmission line parameter is not negligible, at the moment, the polar current differential protection of the direct-current transmission line is constructed by using the Bergeron equivalent model, and the calculation precision is influenced due to the fact that the frequency-varying characteristic of the line parameter is not considered, so that the differential protection performance is reduced.
Calculating a line mode difference current and a ground mode difference current at a midpoint (reference point) r of a line according to the Bergeron model, constructing an action criterion of the differential protection of the direct-current transmission line by using the line mode difference current at the reference point r as a differential current, and then constructing a fault pole selection criterion by using a ratio of the ground mode difference current at the reference point r to the line mode difference current.
The method comprises the following steps:
as shown in fig. 1, a transmission line of a high-voltage direct-current transmission system is generally composed of two transmission lines, namely a positive transmission line and a negative transmission line. In the figure, uJP、uJN、iJP、iJNRespectively collecting positive and negative voltage and current of a converter station J outlet measuring device; u. ofKP、uKN、iKP、iKNRespectively positive and negative voltage and current collected by a converter station K outlet measuring device. The electrical coupling exists between the positive and negative electrical quantities, and the positive and negative voltages can be decoupled by adopting a decoupling matrix S shown in formula (1).
And obtaining mutually independent line mode components and earth mode components after decoupling. The line mode component forms a loop through a positive and negative polar line, and the earth mode component reflows through an earth loop. Since the line mode component is not influenced by the skin effect of the earth loop, the frequency variation phenomenon of the line parameter is obviously lighter than that of the earth mode component. The travelling wave dispersion phenomenon caused by the line parameter frequency change characteristic is a main reason for reducing the calculation accuracy of the Bergeron equivalent model. Fig. 2 shows a comparison of the linear mode wave velocity and the ground mode wave velocity in the normal case. It can be seen that the linear mode wave velocity changes little with frequency, and is closer to the light velocity in vacuum no matter in the low frequency band or the high frequency band; the frequency-dependent characteristic of the ground mode wave velocity is quite obvious, the size of the ground mode wave velocity in a low frequency band is only slightly higher than half of the light velocity in vacuum, and the ground mode wave velocity gradually increases along with the increase of the frequency. Therefore, the line mode component of the line can still ensure high calculation precision of the Bergeron equivalent model.
(1) The decoupling matrix S of the formula (1) is utilized to collect voltage and current sampling values u at two ends of the line collected by the voltage transformer and the current transformer at the outlets of the converter stations at two sides of the lineJP、uJN、iJP、iJN、uKP、uKN、iKP、iKNDecoupling to obtain voltage and current line mode components u on two sides of the lineJ1、uK1、iJ1、iK1And the earth-mode component uJ0、uK0、iJ0、iK0。
(2) According to the Bergeron equivalent model, the voltage and current line mode component u of the line J side is utilizedJ1、iJ1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesJr1Using the voltage-current line modulus component u on the line K sideK1、iK1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesKr1Line mode differential current i of reference point1diffAs the difference between them:
i1diff＝isjr1-iskr1(2)
similarly, the earth mode difference current i of the reference point can be calculated0diff。
(3) Because the traveling wave dispersion phenomenon of the line mode component of the direct current transmission line is slight, the calculation precision of the Bergeron model is high, and the line mode difference current of the reference point is used for constructing the action criterion of the current differential protection of the direct current transmission line:
in the formula isetThreshold value, max { i, for current differential protection criterion1unbalanceDenotes the maximum value of unbalance current that may occur in the differential current of the bereay line model in various out-of-zone fault situations (in general, the value may be taken as the unbalance current that occurs in the differential protection in the case of a metallic fault between the near-end bipolar electrodes outside the protection zone), krelThe reliability coefficient is generally 1.3-1.5.
(4) The current differential protection of the direct-current transmission line is constructed by utilizing the line mode difference current, so that the differential protection loses the natural fault pole selection capability of the differential protection, and a fault pole selection criterion needs to be additionally set. The fault pole selection criterion is still considered to be constructed using the modulus difference current information at the reference point r. Because the action of the fault pole selection criterion is after the action of the current differential protection, only the judgment of the fault type is needed to be realized, and the influence of the line parameter frequency conversion phenomenon on the calculation accuracy of the Bergeron equivalent model can be properly relaxed. If the dispersion phenomenon caused by the frequency-dependent characteristics of the direct-current transmission line parameters is not considered, the ground mode difference current and the line mode difference current of the reference point satisfy the following formula:
in the formula if0、if1A ground mode component and a line mode component of the fault current, respectively; i.e. ifP、ifNThe fault currents injected into the fault point by the positive line and the negative line are respectively.
Analyzing the relation between the modulus difference current of the reference point and the fault type:
a) as shown in fig. 3(a), the fault currents injected into the fault point by the positive electrode line and the negative electrode line during the inter-bipolar fault are related as follows:
by substituting equation (5) for equation (4), the relationship between the modulus difference current at the reference point at the time of the bipolar electrode fault and the fault current can be obtained as follows:
the ratio of the two is 0.
b) As shown in fig. 3(b), the fault current injected into the fault point by the positive line and the negative line during the positive ground fault has the following relationship:
by substituting equation (7) for equation (4), the relationship between the modulus difference current of the reference point and the fault current when the positive electrode is in the ground fault can be obtained as follows:
the ratio of the two is 1.
c) As shown in fig. 3(b), the fault current injected into the fault point by the positive line and the negative line at the time of the negative ground fault has the following relationship:
by substituting equation (10) into equation (4), the relationship between the modulus difference current of the reference point and the fault current at the time of the negative ground fault can be obtained as follows:
the ratio of the two is-1.
Accordingly, the fault pole selection criterion can be designed and constructed by utilizing the ratio of the earth mode difference current and the line mode difference current at the reference point:
in the formula, m is a threshold value, and sufficient sensitivity can be ensured for judging various fault types when a certain margin is considered to be set as 0.1. After the current differential protection criterion of the formula (3) acts, the fault type can be judged by using the formula (9).
The invention designs a modulus-based direct current transmission line current differential protection method and a matched fault type judgment method thereof, aiming at the problem of poor current differential protection action characteristics of a direct current transmission line. The method fully considers the distribution characteristic and the frequency variation characteristic of the power transmission line parameters, constructs the protection criterion of current differential by utilizing the line mode differential current of a Bergeron model calculation reference point, and simultaneously eliminates the influence of the distributed capacitance current and the influence of the traveling wave dispersion phenomenon on the differential current calculation precision; in addition, the fault pole selection criterion ensures the fault pole selection capability of the current differential protection based on the modulus information. The invention thoroughly eliminates the problem that the traditional direct current transmission current differential protection needs long-delay locking to avoid unbalanced current, improves the quick action, sensitivity and reliability of the current differential protection, and is also suitable for the (special) high-voltage direct current transmission line with longer transmission distance and more obvious traveling wave dispersion phenomenon.
Claims (3)
1. A differential protection method for a long-distance high-voltage direct-current transmission line is provided, wherein two ends of the transmission line are a J side and a K side, and a midpoint r of the line is taken as a reference point, and comprises the following steps:
(1) let uJP、uJN、iJP、iJNRespectively collecting positive and negative voltage and current of a converter station J outlet measuring device; u. ofKP、uKN、iKP、iKNRespectively collecting positive and negative voltage and current values of a voltage and current sampling value u at two ends of the line collected by a voltage transformer and a current transformer at the outlets of the converter stations at two sides of the lineJP、uJN、iJP、iJN、uKP、uKN、iKP、iKNDecoupling is carried out by adopting a decoupling matrix S shown as the following formula to obtain voltage and current line mode components u on two sides of the lineJ1、uK1、iJ1、iK1And the earth-mode component uJ0、uK0、iJ0、iK0The line mode component forms a loop through a positive and negative two-pole line, and the earth mode component flows back through an earth loop:
(2) according to the Bergeron equivalent model, the voltage and current line mode component u of the line J side is utilizedJ1、iJ1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesJr1Using the voltage-current line modulus component u on the line K sideK1、iK1Calculating to obtain a line mode calculated current value i at the midpoint r of the linesKr1Line mode differential current i of reference point1diffAs the difference between them:
i1diff＝isjr1-iskr1
similarly, the earth mode difference current i of the reference point can be calculated0diff；
(3) Constructing an action criterion of the current differential protection of the direct-current transmission line by using the line mode difference current of the reference point:
in the formula isetThreshold value, max { i, for current differential protection criterion1unbalanceDenotes the maximum unbalance current value, k, of the differential current of the Bergeron mode under various out-of-region fault conditionsrelIs a reliability factor;
(4) and constructing a fault pole selection criterion by using the ratio of the earth mode difference current and the line mode difference current at the reference point:
in the formula, m is a threshold value, and sufficient sensitivity can be ensured for judging various fault types when a certain margin is considered to be set as 0.1.
2. The differential protection method for the long-distance high-voltage direct current transmission line according to claim 1, wherein max { i } is1unbalanceAnd the value is the unbalanced current of the differential protection when the metallic fault occurs between the near-end bipolar electrodes outside the protection area.
3. The differential protection method for the long-distance high-voltage direct current transmission line according to claim 1, wherein k is krelTaking 1.3-1.5.
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