CN110350493B - Medium-voltage flexible direct-current system fault detection method based on line current second-order derivative - Google Patents

Medium-voltage flexible direct-current system fault detection method based on line current second-order derivative Download PDF

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CN110350493B
CN110350493B CN201910554408.6A CN201910554408A CN110350493B CN 110350493 B CN110350493 B CN 110350493B CN 201910554408 A CN201910554408 A CN 201910554408A CN 110350493 B CN110350493 B CN 110350493B
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CN110350493A (en
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王盼宝
孙红梅
郝鑫
任鹏
王卫
徐殿国
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Harbin Institute of Technology
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    • 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
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
    • 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
    • G01R31/088—Aspects of digital computing
    • 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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Abstract

The invention provides a line current second derivative-based medium-voltage flexible direct current system fault detection method, which comprises four criteria: the method comprises the following steps of judging a fault triggering criterion, a fault side criterion, a fault type criterion and a fault line polarity criterion, and realizing the purposes of detecting a direct current fault, identifying a fault side, judging the fault type and identifying the fault line polarity by utilizing the four criteria, wherein the direct current fault detection and the fault side identification are realized by utilizing a unipolar line current second-order derivative value, and the fault type judgment and the fault line polarity identification are realized by a difference value of positive and negative line current second-order derivative values of the fault side. The fault detection method provided by the invention is based on the system parameters measured locally, does not need communication, almost has no time delay, and has certain practical prospect.

Description

Medium-voltage flexible direct-current system fault detection method based on line current second-order derivative
Technical Field
The invention belongs to the technical field of fault detection, and particularly relates to a fault detection method of a medium-voltage flexible direct-current system based on a line current second-order derivative.
Background
The Voltage Source Converter type (Voltage Source Converter) flexible direct current transmission technology based on the full-control type device and the Pulse Width Modulation (PWM) technology has the characteristics of being capable of quickly and flexibly controlling power, improving system stability and the like, and has the characteristics that when the system power flow is reversed, the direction of direct current is reversed, the polarity of direct current Voltage is unchanged, a multi-terminal direct current system is easy to form and the like. With the increasing shortage of land resources such as power transmission corridors and the rapid development of renewable energy power generation, the technology is applied to the fields of renewable energy power generation grid connection, multi-terminal direct current power transmission, power transmission and distribution systems, ship power supply systems and the like in the global range. The direct current fault protection of the medium-voltage flexible direct current system is one of key technologies for development, and main technical difficulties include reliable identification and rapid isolation of a fault line. Due to the low damping of the flexible dc network, after a dc fault occurs, the dc fault current will rise at a very fast rate, typically reaching several tens of times the rated current within 10ms after the fault. Compared with an alternating current system, a direct current has no natural zero crossing point, so that the conventional alternating current circuit breaker is difficult to break a fault current. If not isolated in time, the fault current threatens the normal operation of the equipment in the whole system. Therefore, there is a need for a fast and reliable fault detection and isolation method for medium voltage flexible dc systems.
Aiming at the problem of direct current fault detection of a flexible direct current system, a student provides a method for positioning and isolating a fault direct current line by using a handshake method and recovering the fault direct current line under the condition of no need of communication, but the method has the problem of short-time power failure in a non-fault part in the identification process and influences the reliable power supply of the system. In addition, the scholars propose to apply the wavelet transform to the traveling wave differential protection, and adopt the wavelet transform to extract the wavelet transform modulus maximum of the traveling wave to form the differential protection criterion.
Disclosure of Invention
The invention aims to realize rapid detection of a direct-current fault of a medium-voltage flexible direct-current system, and provides a fault detection method of the medium-voltage flexible direct-current system based on a second derivative of a line current.
The invention is realized by the following technical scheme, and provides a medium-voltage flexible direct-current system fault detection method based on a line current second-order derivative, which comprises four criteria: the method comprises the following steps that four criteria are utilized to achieve the purposes of detecting a direct-current fault, identifying a fault side, judging a fault type and identifying the polarity of a fault line, wherein the direct-current fault detection and the fault side identification are achieved by utilizing a unipolar line current second-order derivative value, and the fault type judgment and the fault line polarity identification are achieved by the difference value of positive and negative line current second-order derivative values of the fault side;
presetting a fault trigger threshold, a fault side identification threshold, a fault type identification threshold and a fault line polarity identification threshold; when the absolute value of the second derivative value of the line current at any end is detected to exceed a fault trigger threshold, judging that the system has a direct current fault; further comparing the absolute value of the second derivative value of the line current at each end with a fault side identification threshold value to determine a fault side; after the fault side is determined, comparing the absolute value of the absolute value difference of the second-order derivative of the current of the positive and negative electrode lines of the fault side with a fault type identification threshold, if the absolute value difference is small, determining that the fault side is an inter-electrode short-circuit fault, otherwise, determining that the fault side is a single-pole grounding fault; if the fault is a single-pole ground fault, further comparing the difference value of the absolute values of the second-order derivatives of the currents of the positive and negative lines at the fault side with the fault line polarity identification threshold, if the difference value is larger than the fault line polarity identification threshold, determining that the fault line is a positive-pole ground fault, otherwise, determining that the fault line is a negative-pole ground fault.
Further, the fault triggering criterion is specifically:
when the system stably operates, the line current is a stable constant under the ideal condition, the first derivative and the second derivative are 0, when a fault occurs, the second derivative of the direct current line current generates sudden change, and the fault triggering criterion is as follows:
Figure BDA0002106450460000021
wherein iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000022
and
Figure BDA0002106450460000023
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH1(set)Triggering a threshold value for a preset fault;
when the system is operated safely, the second derivative of the line current under the ideal condition should be 0, the influence of noise and harmonic interference is actually considered, the fault triggering criterion should be a value greater than zero, and the following two conditions should be satisfied:
Figure BDA0002106450460000024
namely I'TH1(set)The threshold value is selected such that it is greater than the second derivative of the line currents for stable operation and for consideration of disturbances, and less than the second derivative of the line currents on either side of the fault time.
Further, the fault-side criterion is specifically:
when a direct-current fault occurs, the current at the fault side suddenly changes, and the current change signal is transmitted to the non-fault side only through the line reactance and the current limiting inductor at the non-fault side, so that the second derivative of the line current at the non-fault side is smaller than that of the line current at the fault side, and the closer the fault position is to the fault side, the larger the difference value of the second derivatives of the line current at the fault side and the non-fault side is; the fault side criterion is as follows:
Figure BDA0002106450460000031
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000032
and
Figure BDA0002106450460000033
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH2(set)Identifying a threshold value for a preset fault side, and when the absolute value of the second derivative value of the line current is greater than the threshold value, determining that the direct current side of the converter station has a fault;
the second derivative of the line current at the fault side is far larger than that at the non-fault side at the fault moment, so that
Figure BDA0002106450460000034
Wherein iLkThe maximum value of the second derivative of the current of the three-terminal line at the fault moment is obtained;
the fault side criterion should satisfy the condition:
Figure BDA0002106450460000035
in which I, k represent the I, k sides of the converter station, i.e. I "TH2(set)The threshold value is selected to be smaller than the second-order derivative value of the line current of the fault side and larger than the second-order derivative value of the line current of the non-fault side, so that the fault side can be effectively identified.
Further, the fault type criterion is specifically:
the absolute value of the absolute value difference of the second derivative of the current of the positive and negative lines on the fault side is used as the fault type criterion:
Figure BDA0002106450460000036
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000037
and
Figure BDA0002106450460000038
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH3(set)And identifying a threshold value for a preset fault type, and if the absolute value of the absolute value difference of the second derivative of the current of the positive and negative electrode lines is detected to be greater than the threshold value, judging that the fault is a single-pole grounding fault, otherwise, judging that the fault is an inter-pole short-circuit fault.
Further, the fault line polarity criterion is specifically:
when a single-pole ground fault occurs, the second derivative of the current of the fault pole line is larger than that of the current of the non-fault pole line, and the polarity criterion of the fault line is as follows:
Figure BDA0002106450460000041
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000042
and
Figure BDA0002106450460000043
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH4(set)And identifying a threshold value for the preset polarity of the fault line, if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is greater than the threshold value, judging that the positive electrode is in ground fault, and if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is less than the opposite number of the threshold value, judging that the negative electrode is in ground fault.
The invention has the beneficial effects that: the fault detection method provided by the invention is based on the system parameters measured locally, does not need communication, almost has no time delay, and has certain practical prospect.
Drawings
FIG. 1 is a flow chart of a method for detecting faults of a medium voltage flexible direct current system based on a second derivative of a line current according to the present invention;
FIG. 2 is a block diagram of a three-terminal VSC based medium voltage flexible DC system; wherein U iss1-Us3For AC mains voltage, L1-L3For the converter reactor, Lc1-Lc3For line current-limiting inductance, C1-C3Is a DC side capacitor, Rd1-Rd3Is a line equivalent resistance, Ld1-Ld3The line equivalent inductance is obtained;
FIG. 3 is a fault equivalent circuit diagram of a medium voltage flexible DC system; wherein VSC1 denotes the converter station 1, UdcIs a DC side capacitorPressure, Lc1pCurrent-limiting inductance, L, for the series connection of the positive direct-current line on the 1 side of the converter stationc1nCurrent-limiting inductance, R, for the series connection of the negative DC line on the 1 side of the converter stationfIs a fault resistance;
FIG. 4 is a simulation waveform diagram of the second derivative of the line current in the case of a DC fault.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
With reference to fig. 1, the present invention provides a method for detecting a fault of a medium voltage flexible dc system based on a second derivative of a line current, where the method includes four criteria: the method comprises the following steps that four criteria are utilized to achieve the purposes of detecting a direct-current fault, identifying a fault side, judging a fault type and identifying the polarity of a fault line, wherein the direct-current fault detection and the fault side identification are achieved by utilizing a unipolar line current second-order derivative value, and the fault type judgment and the fault line polarity identification are achieved by the difference value of positive and negative line current second-order derivative values of the fault side;
presetting a fault trigger threshold, a fault side identification threshold, a fault type identification threshold and a fault line polarity identification threshold; when the absolute value of the second derivative value of the line current at any end is detected to exceed a fault trigger threshold, judging that the system has a direct current fault; further comparing the absolute value of the second derivative value of the line current at each end with a fault side identification threshold value to determine a fault side; after the fault side is determined, comparing the absolute value of the absolute value difference of the second-order derivative of the current of the positive and negative electrode lines of the fault side with a fault type identification threshold, if the absolute value difference is small, determining that the fault side is an inter-electrode short-circuit fault, otherwise, determining that the fault side is a single-pole grounding fault; if the fault is a single-pole ground fault, further comparing the difference value of the absolute values of the second-order derivatives of the currents of the positive and negative lines at the fault side with the fault line polarity identification threshold, if the difference value is larger than the fault line polarity identification threshold, determining that the fault line is a positive-pole ground fault, otherwise, determining that the fault line is a negative-pole ground fault. And finally, identifying the direct-current fault, the fault side, the fault type and the fault line polarity of the medium-voltage flexible direct-current system.
The invention provides a direct current fault rapid identification method based on a line current second derivative based on a medium voltage flexible direct current system (VSC-MVDC) which is added with a direct current line current-limiting inductor and is based on three-terminal VSC, wherein a system topological structure is shown in figure 2.
Two typical dc faults in the medium voltage flexible dc system are a single-pole ground fault and an inter-pole short-circuit fault, which are analyzed by taking the converter station 1 as an example, and equivalent circuits of the single-pole ground fault (taking the positive ground fault as an example) and the inter-pole short-circuit fault are shown in fig. 3(a) and 3 (b). Because the IGBT in the practical engineering has a reliable self-protection function, the IGBT can be immediately turned off after the direct-current fault occurs, and the converter is supposed to stand and be locked after the direct-current fault occurs. Due to the special structural characteristics of the inertia of the VSC-MVDC system and the parallel capacitance of the direct current side, the fault characteristics of the direct current line are in a stage characteristic. The first stage of the unipolar ground fault and the interelectrode short-circuit fault is a capacitor discharge stage, the converter IGBT is turned off at the stage, the alternating current side current is not increased, the direct current side capacitor voltage is attenuated, and the direct current is increased. In order to ensure that the rapid protection action and the tripping of the direct current breaker are before the zero-crossing moment of oscillation of the direct current capacitor voltage, the direct current fault needs to be identified at the stage.
The direction shown in FIG. 3 is defined as the positive direction, where Udc1Is the DC side voltage, i is the line current, Lc1Is a current-limiting inductor, RfAs fault resistance, Rd1And Ld1The equivalent resistance and reactance of the pi-type direct-current line equivalent circuit are respectively considered, and the influence of the equivalent capacitive reactance of the direct-current line is ignored considering that the capacitance C on the direct-current side is far larger than the parallel capacitance of the pi-type equivalent circuit.
With reference to fig. 3(a), the influence of the coupling of the non-fault-side current is ignored, and the fault characteristics at the capacitor discharge stage after the inter-electrode short-circuit fault occurs are analyzed. At this stage, the dc side capacitor, the current-limiting inductor, and the line equivalent reactance form an RLC loop, which can be obtained from KVL:
Figure BDA0002106450460000061
in the formula L1Current limiting inductance L for fault side circuitc1Equivalent inductance L of direct current line between fault side converter station and fault positiond1And, R1For the equivalent resistance R of the direct current line between the fault side converter station and the fault positiond1And a fault resistance RfThe sum of (1).
Wherein:
Figure BDA0002106450460000062
in the formula C1D represents a differential operation for the dc side capacitance.
When in use
Figure BDA0002106450460000063
The solution to differential equation (1) can be obtained as an under damped response:
Figure BDA0002106450460000064
further, the first derivative and the second derivative expression of the line current with respect to time in the under-damped response can be obtained:
Figure BDA0002106450460000065
Figure BDA0002106450460000066
wherein
Figure BDA0002106450460000067
L1＝Lc1+Ld1，R1＝Rd1+Rf，
Figure BDA0002106450460000068
A1＝I01，
Figure BDA0002106450460000069
U01And I01The initial values of the dc side voltage and the dc line current, respectively.
Recording fault time t0The second derivative of the line current at the fault instant is given by equation (5) as 0:
Figure BDA00021064504600000610
when in use
Figure BDA00021064504600000611
The solution of differential equation (1) is the over-damped response:
Figure BDA00021064504600000612
further, the first derivative and the second derivative expression of the line current with respect to time in the over-damped response can be obtained:
Figure BDA00021064504600000613
Figure BDA00021064504600000614
wherein the content of the first and second substances,
Figure BDA0002106450460000071
m2＝I01-m1，
Figure BDA0002106450460000072
the second derivative of the line current at the time of the fault can be obtained from equation (9):
Figure BDA0002106450460000073
as can be seen from the formulas (6) and (10), for
Figure BDA0002106450460000074
Under-damped response under circumstances and
Figure BDA0002106450460000075
and under the condition of over-damping response, the second derivative value of the line current at the fault moment is the same.
With reference to fig. 3(b), the influence of the coupling of the non-fault-side current is ignored, and the fault characteristics at the capacitor discharge stage after the positive ground fault are analyzed. At this stage, the dc side capacitor, the current-limiting inductor, and the line equivalent reactance form an RLC loop, which can be obtained from KVL:
Figure BDA0002106450460000076
wherein:
Figure BDA0002106450460000077
from the above analysis, it is known that, under the influence of neglected coupling, when the anode ground fault resistance is 2 times of the inter-electrode short-circuit fault, the anode line current under the anode ground fault is the same as the line current analysis condition under the inter-electrode short-circuit fault, but in comparison, the faultless line current under the anode ground fault is less influenced, and the anode line and the cathode line are equally influenced when the inter-electrode short-circuit fault is caused, so that the influence range is wider than that of the anode ground fault, and the system is more harmful.
It can be seen that, when a direct current fault occurs, the first derivative and the second derivative of the line current are both subjected to sudden change, and the second derivative of the direct current line current is selected as a fault criterion to detect the direct current fault in consideration of the inhibiting effect of the current limiting inductor in the system on the line current change rate after the fault and the inhibiting effect of the line inductor on the line current change rate along with the increase of the fault distance.
The fault triggering criterion is specifically as follows:
when the system stably operates, the line current is a stable constant under the ideal condition, the first derivative and the second derivative are 0, when a fault occurs, the second derivative of the direct current line current generates sudden change, and the fault triggering criterion is as follows:
Figure BDA0002106450460000078
wherein iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000079
and
Figure BDA00021064504600000710
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH1(set)Triggering a threshold value for a preset fault;
when the system is operated safely, the second derivative of the line current under the ideal condition should be 0, the influence of noise and harmonic interference is actually considered, the fault triggering criterion should be a value greater than zero, and the following two conditions should be satisfied:
Figure BDA0002106450460000081
namely I'TH1(set)The threshold value is selected such that it is greater than the second derivative of the line currents for stable operation and for consideration of disturbances, and less than the second derivative of the line currents on either side of the fault time.
As long as the second derivative of the line current on one side is detected to be greater than the fault trigger threshold I during system operation "TH1(set)And the system can be judged to be in fault. When a fault occurs at a common intersection point of the three ends, the distance from the fault position to the three ends is the same, the impedance of a capacitor discharge loop is larger, and the line currents are different when the three ends are in a stable state, so that the second-order derivatives of the line currents of the three ends are not completely the same under the influence of the fault, but the comprehensive degree of the influence of the fault on the three ends is the minimum at the moment, and the fault trigger threshold is set by taking the minimum value of the second-order derivatives of the line currents of the three ends as the reference when. From equation (6), the second derivative amplitude of the line current at the fault is affected by the fault resistance, so the fault resistance is taken into account when selecting the threshold. The fault resistance is not very large when the interelectrode short circuit occurs, and the high impedance state is presented when the monopolar grounding fault occurs, so that the fault resistance is set to be 1 omega when the interelectrode short circuit fault occurs at the three-terminal common intersection point, and the fault resistance is set to be 100 omega when the monopolar grounding fault occurs, in combination with I'TH1(set)The conditions to be met are set as follows:
Figure BDA0002106450460000082
in the formula Krel1The setting coefficient is triggered for faults, and the value of the setting coefficient is less than 1 for ensuring reliable protection action, wherein the value is 0.6.
The fault side criterion is specifically as follows:
when a direct-current fault occurs, the current at the fault side suddenly changes, and the current change signal is transmitted to the non-fault side only through the line reactance and the current limiting inductor at the non-fault side, so that the second derivative of the line current at the non-fault side is smaller than that of the line current at the fault side, and the closer the fault position is to the fault side, the larger the difference value of the second derivatives of the line current at the fault side and the non-fault side is; the fault side criterion is as follows:
Figure BDA0002106450460000091
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000092
and
Figure BDA0002106450460000093
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH2(set)Identifying a threshold value for a preset fault side, and judging that the direct current side of the converter station has a fault when the absolute value of the second derivative value of the line current is greater than the threshold value;
the second derivative of the line current at the fault side is far larger than that at the non-fault side at the fault moment, so that
Figure BDA0002106450460000094
In the formula iLkRepresenting the maximum value of the second derivative of the three-terminal line current at the moment of the fault.
The fault side criterion should satisfy the condition:
Figure BDA0002106450460000095
in which I, k represent the I, k sides of the converter station, i.e. I "TH2(set)The threshold value is selected to be smaller than the second-order derivative value of the line current of the fault side and larger than the second-order derivative value of the line current of the non-fault side, so that the fault side can be effectively identified.
And the second derivative of the line current obtained under the direct current fault which is set in the fault triggering criterion setting and occurs at the three-terminal common intersection point is taken as the reference to identify the fault sideThreshold value I "TH2(set)Setting:
Figure BDA0002106450460000096
in the formula Krel2And the fault side setting coefficient is obtained. Because the second derivative of the line current at the fault side is larger than that of the line current at the non-fault side, for any direct current fault which does not occur at the three-terminal common intersection point, the second derivative of the line current at the fault side is inevitably larger than that of the line current at any side when the direct current fault occurs at the three-terminal common intersection point. Considering the reference second derivative value as the second derivative of the line current under the harsher working condition, Krel2May be between 0.95 and 1.05, where the value is 1.
The fault type criterion is specifically as follows:
for unipolar grounding and interelectrode short-circuit faults, the second derivative of the fault pole line current at the moment of the fault is almost the same, and effective distinction can not be made only by the second derivative of the fault pole line current. Considering that the positive and negative lines are affected by almost the same effect when an interelectrode fault occurs, and the effect of the fault pole is much larger than that of a non-fault pole when a single-pole ground fault occurs, the absolute value of the absolute value difference of the second derivative of the current of the positive and negative lines on the fault side can be used as the fault type criterion:
Figure BDA0002106450460000101
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000102
and
Figure BDA0002106450460000103
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH3(set)And identifying a threshold value for a preset fault type, and if the absolute value of the absolute value difference of the second derivative of the current of the positive and negative electrode lines is detected to be greater than the threshold value, judging that the fault is a single-pole grounding fault, otherwise, judging that the fault is an inter-pole short-circuit fault.
From the above analysis, it is known that, for the distinction between the single-pole ground fault and the inter-pole short-circuit fault, the second derivative of the fault pole line current can not be used alone, and the difference of the amplitudes of the second derivatives of the fault side positive and negative pole line currents is further proposed to be used as the fault type criterion. Theoretically, the difference of the second derivative amplitudes of the currents of the positive and negative lines is 0 when the interelectrode is in short circuit, the influence of interference such as noise and harmonic waves is considered, when a single-pole high-impedance ground fault with the fault resistance of 100 omega occurs at a common intersection point of the three ends, the minimum value of the second derivative amplitudes of the currents of the three ends is taken as a reference, and a fault type identification threshold I' is identified "TH3(set)The following settings are performed:
Figure BDA0002106450460000104
in the formula Krel3For the fault type setting coefficient, 0.6 is taken here. After the fault type is effectively identified, if the fault type is identified to be a single-pole grounding fault, the polarity of the fault line needs to be further judged.
The fault line polarity criterion is specifically as follows:
when a single-pole ground fault occurs, the second derivative of the current of the fault pole line is larger than that of the current of the non-fault pole line, and the polarity criterion of the fault line is as follows:
Figure BDA0002106450460000111
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure BDA0002106450460000112
and
Figure BDA0002106450460000113
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH4(set)And identifying a threshold value for the preset polarity of the fault line, if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is greater than the threshold value, judging that the positive electrode is in ground fault, and if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is less than the opposite number of the threshold value, judging that the negative electrode is in ground fault.
When the positive pole earth fault occurs in the system, the difference of the current second derivative amplitudes of the positive and negative pole lines at the fault side is a positive value and is close to the value of the current second derivative amplitude of the positive pole line, and for the negative pole earth fault, the difference of the current second derivative amplitudes of the positive and negative pole lines at the fault side is a negative value and is close to the value of the second derivative amplitude of the current of the negative pole line. Still taking the minimum value of the second derivative amplitude of the line current when the unipolar high-impedance fault occurs at the three-end common intersection as the reference to identify the threshold value I' of the polarity of the fault line "TH4(set)The following settings are performed:
Figure BDA0002106450460000114
in the formula Krel4For the polar setting coefficient of the fault line, under the condition of considering interference, when a single-pole earth fault occurs, the value of the second derivative of the current of the non-fault polar line is not 0, the absolute value of the difference of the amplitudes of the second derivatives of the current of the fault pole and the current of the non-fault polar line is lower than the amplitude of the second derivative of the current of the fault polar line, and K is taken hererel4Is 0.8.
In order to verify the effectiveness of the method for detecting the direct current fault of the medium-voltage flexible direct current system based on the second derivative of the line current, a three-terminal VSC-MVDC system model shown in FIG. 2 is established based on MATLAB simulation software, wherein a converter station 1 is controlled by constant direct current voltage, and converter stations 2 and 3 are controlled by constant active power. The direct current line is equivalent by adopting an RL model, and simulation parameters are shown in Table 1.
TABLE 1 System simulation parameters
Figure BDA0002106450460000115
Figure BDA0002106450460000121
According to the threshold selection method, the obtained fault detection thresholds are respectively as follows: fault triggering threshold I'TH1(set)＝1.5442×108kA/(ms)2Fault side identification threshold I "TH2(set)＝2.5737×108kA/(ms)2Fault type recognition threshold I "TH3(set)＝3.0981×109kA/(ms)2Identification threshold value I for fault line polarity "TH4(set)＝4.1308×109kA/(ms)2. When a direct current line fault occurs at the converter station 1 side, the fault distance is set to be 50km, and the second derivative waveforms of the line current are shown in fig. 4(a) and 4(b) when the positive ground fault resistance is 0 Ω and 20 Ω respectively; the second derivative waveform of the line current at an interelectrode short-circuit fault resistance of 0 Ω is shown in fig. 4 (c).
It can be seen that when a direct-current fault occurs at the converter station 1 side, the second derivative of the line current at the fault side generates a sudden change, so that the second derivative of the line current can be used as a fault criterion. For an interelectrode short-circuit fault, the second derivative of the line current at the fault moment is far larger than the second derivative of the line current at the non-fault moment; for positive ground faults, the positive line current second derivative is much larger at the moment of the fault than the negative line current second derivative. The direct current fault detection method based on the second derivative of the line current can realize the rapid identification of the direct current fault, and the fault detection result is shown in table 2.
Table 2 DC Fault detection results of the inventive method under different working conditions
Figure BDA0002106450460000122
As can be seen from table 2, after the inter-electrode short-circuit fault and the single-pole ground fault occur, the second derivative of the fault-side direct-current line current is largeAt fault detection threshold 1.5442 x 108kA/(ms)2The occurrence of a failure can be detected quickly. The fault side dc line current second derivative is then compared to a fault side identification threshold 2.5737 x 108kA/(ms)2By making a comparison, it can be determined that a fault has occurred at the converter station 1 side. Then, for the short circuit fault between the electrodes, the difference of the current second derivative amplitudes of the positive and negative electrode lines at the fault side is calculated to be 0 and lower than the fault type identification threshold value 3.0981 multiplied by 109kA/(ms)2And judging that an interelectrode short-circuit fault occurs on a side line of the system converter station 1. For single-pole grounding fault, the difference of the amplitudes of the second derivatives of the currents of the positive and negative electrode lines is 6.3574 × 1010kA/(ms)2And 4.9474 × 1010kA/(ms)2Above fault type identification threshold 3.0981 x 109kA/(ms)2Judging as a single-pole ground fault; finally, 6.3574 × 1010kA/(ms)2And 4.9474 × 1010kA/(ms)2And fault line polarity determination threshold 4.1308 x 109kA/(ms)2And comparing, and judging that the positive grounding fault occurs on the side line of the system converter station 1.
The method for detecting the faults of the medium-voltage flexible direct-current system based on the second-order derivative of the line current can realize the rapid detection of the direct-current faults in the medium-voltage flexible direct-current system.
The flow of the fault detection method based on the second derivative of the line current provided by the invention is shown in fig. 1. Firstly, the line current of the system is collected, the second derivative modulus value is calculated, and the absolute value of the second derivative and a fault triggering threshold value I'TH1(set)Making a comparison if the value is less than I "TH1(set)If the system has no direct current fault, otherwise, judging that the system has the direct current fault; further relating the absolute value of the second derivative of the line current to a fault side identification threshold I'TH2(set)For comparison, greater than I "TH2(set)A direct current fault occurs at one side of the switch; further calculating the absolute value of the difference value of the absolute values of the second derivatives of the positive and negative line currents on the fault side and comparing the absolute value with a fault type identification threshold value I'TH3(set)Making a comparison, if less than I "TH3(set)Then it is determined as an inter-electrode short circuitOtherwise, the fault is a single-pole grounding fault; if the fault is judged to be a single-pole grounding fault, further calculating the difference value of the absolute values of the second derivatives of the currents of the positive and negative lines at the fault side and the polarity identification threshold I' of the fault line "TH4(set)Making a comparison, if greater than I "TH4(set)The positive pole is grounded, otherwise, the negative pole is grounded. Finally, the direct-current fault in the medium-voltage flexible direct-current system can be quickly detected, and the fault side, the fault type and the fault line polarity of the single-pole ground fault can be effectively identified.
The method for detecting the fault of the medium-voltage flexible direct-current system based on the second derivative of the line current provided by the invention is described in detail, a specific example is applied in the method for explaining the principle and the implementation mode of the invention, and the description of the embodiment is only used for helping to understand the method and the core idea of the invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.

Claims (5)

1. The method for detecting the faults of the medium-voltage flexible direct-current system based on the second derivative of the line current is characterized by comprising the following steps: the method comprises four criteria: the method comprises the following steps that four criteria are utilized to achieve the purposes of detecting a direct-current fault, identifying a fault side, judging a fault type and identifying the polarity of a fault line, wherein the direct-current fault detection and the fault side identification are achieved by utilizing a unipolar line current second-order derivative value, and the fault type judgment and the fault line polarity identification are achieved by the difference value of positive and negative line current second-order derivative values of the fault side;
presetting a fault trigger threshold, a fault side identification threshold, a fault type identification threshold and a fault line polarity identification threshold; when the absolute value of the second derivative value of the line current at any end is detected to exceed a fault trigger threshold, judging that the system has a direct current fault; further comparing the absolute value of the second derivative value of the line current at each end with a fault side identification threshold value to determine a fault side; after the fault side is determined, comparing the absolute value of the absolute value difference of the second-order derivative of the current of the positive and negative electrode lines of the fault side with a fault type identification threshold, if the absolute value difference is small, determining that the fault side is an inter-electrode short-circuit fault, otherwise, determining that the fault side is a single-pole grounding fault; if the fault is a single-pole ground fault, further comparing the difference value of the absolute values of the second-order derivatives of the currents of the positive and negative lines at the fault side with the fault line polarity identification threshold, if the difference value is larger than the fault line polarity identification threshold, determining that the fault line is a positive-pole ground fault, otherwise, determining that the fault line is a negative-pole ground fault.
2. The method of claim 1, wherein: the fault triggering criterion is specifically as follows:
when the system stably operates, the line current is a stable constant under the ideal condition, the first derivative and the second derivative are 0, when a fault occurs, the second derivative of the direct current line current generates sudden change, and the fault triggering criterion is as follows:
Figure FDA0002870581810000011
wherein iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure FDA0002870581810000012
and
Figure FDA0002870581810000013
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH1(set)Triggering a threshold value for a preset fault;
when the system is operated safely, the second derivative of the line current under the ideal condition should be 0, the influence of noise and harmonic interference is actually considered, the fault triggering criterion should be a value greater than zero, and the following two conditions should be satisfied:
Figure FDA0002870581810000014
namely I'TH1(set)The threshold value is selected such that it is greater than the second derivative of the line currents for stable operation and for consideration of disturbances, and less than the second derivative of the line currents on either side of the fault time.
3. The method of claim 1, wherein: the fault side criterion is specifically as follows:
when a direct-current fault occurs, the current at the fault side suddenly changes, and the current change signal is transmitted to the non-fault side only through the line reactance and the current limiting inductor at the non-fault side, so that the second derivative of the line current at the non-fault side is smaller than that of the line current at the fault side, and the closer the fault position is to the fault side, the larger the difference value of the second derivatives of the line current at the fault side and the non-fault side is; the fault side criterion is as follows:
Figure FDA0002870581810000021
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure FDA0002870581810000022
and
Figure FDA0002870581810000023
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH2(set)Identifying a threshold value for a preset fault side, and when the absolute value of the second derivative value of the line current is greater than the threshold value, determining that the direct current side of the converter station has a fault;
the second derivative of the line current at the fault side is far larger than that at the non-fault side at the fault moment, so that
Figure FDA0002870581810000024
Wherein
Figure FDA0002870581810000025
The maximum value of the second derivative of the current of the three-terminal line at the fault moment is obtained;
the fault side criterion should satisfy the condition:
Figure FDA0002870581810000026
in which I, k represent the I, k sides of the converter station, i.e. I "TH2(set)The threshold value is selected to be smaller than the second-order derivative value of the line current of the fault side and larger than the second-order derivative value of the line current of the non-fault side, so that the fault side can be effectively identified.
4. The method of claim 1, wherein: the fault type criterion is specifically as follows:
the absolute value of the absolute value difference of the second derivative of the current of the positive and negative lines on the fault side is used as the fault type criterion:
Figure FDA0002870581810000031
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure FDA0002870581810000032
and
Figure FDA0002870581810000033
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH3(set)Identifying a threshold value for a preset fault type, and if the absolute value difference of the second derivative absolute values of the currents of the positive and negative electrode lines is detectedIf the value is larger than the threshold value, the single-pole grounding fault is judged, otherwise, the inter-pole short-circuit fault is judged.
5. The method of claim 1, wherein: the fault line polarity criterion is specifically as follows:
when a single-pole ground fault occurs, the second derivative of the current of the fault pole line is larger than that of the current of the non-fault pole line, and the polarity criterion of the fault line is as follows:
Figure FDA0002870581810000034
in the formula iLpAnd iLnRespectively the detected current of the positive and negative electrode lines,
Figure FDA0002870581810000035
and
Figure FDA0002870581810000036
respectively the second derivative of the current of the positive and negative lines, I represents the I side of the converter station, and the value range of I is more than or equal to 1 and less than or equal to 3, I'TH4(set)And identifying a threshold value for the preset polarity of the fault line, if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is greater than the threshold value, judging that the positive electrode is in ground fault, and if the difference of the absolute values of the second-order derivatives of the currents of the positive and negative lines is less than the opposite number of the threshold value, judging that the negative electrode is in ground fault.
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