CN110542834A - direct-current power distribution network double-end ranging fault positioning method based on improved injection method - Google Patents
direct-current power distribution network double-end ranging fault positioning method based on improved injection method Download PDFInfo
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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- 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
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- 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
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Abstract
the invention provides a direct-current power distribution network double-end ranging fault positioning method based on an improved injection method, wherein fault ranging modules are arranged at two ends of a direct-current power distribution network, and each fault ranging module comprises a capacitor, an inductor and a single-pole triple-throw switch; the method comprises the steps of adjusting the sizes of a capacitor and an inductor to enable a fault location module and a fault line to form a second-order RLC discharge circuit of underdamped oscillation, determining oscillation frequency and attenuation coefficient of discharge current attenuation by utilizing nonlinear least square fitting, improving a fault location algorithm in a traditional injection method by introducing double-end information, obtaining a fault distance calculation formula with the total length of the line and the inductance and capacitance values of the fault location module as parameters, and eliminating the influence of the inductance and resistance of a direct-current line on fault location. The fault distance measurement module can repeatedly measure, avoids accidental situations, replaces line resistance and inductance with the total line length, has a simple principle, and greatly improves the reliability and accuracy of distance measurement.
Description
Technical Field
the invention relates to the technical field of direct-current power distribution network fault location, in particular to a direct-current power distribution network double-end location fault location method based on an improved injection method.
background
At present, the problems of energy and environment are more prominent, and power generation modes mainly based on clean distributed power sources such as photovoltaic power generation and wind power generation become very important research contents due to the characteristics of cleanness, economy, flexibility and the like, but the intermittent and fluctuating properties of the distributed power sources can affect the stability of the traditional alternating current power grid. In addition, drawbacks of the conventional ac distribution network are gradually revealed, for example, the capacity margin of the ac line does not meet the increasing demand for electricity; the increase of direct current equipment causes the used alternating current-direct current converter to increase the secondary cost of a power grid, and the like, so that the traditional alternating current power distribution network is urgently needed to be reformed. As research on power semiconductor devices in power electronics technology has been rapidly developed, dc power distribution based on voltage source converters can be put into practical use technically. Compared with alternating current power distribution, the direct current power distribution has much smaller electric energy and line loss, and has greater advantages along with the gradual increase of novel direct current equipment. However, the existing direct current distribution system does not have the capacity of clearing line faults, and only carries out fault location after the breaker is disconnected, so that the research on the fault location method of the direct current distribution network has important theoretical significance and practical value for the safe and stable operation of the direct current distribution network.
at present, the existing direct current line fault positioning method at home and abroad is mostly suitable for the condition of high-voltage long-distance direct current transmission, and has larger error for a direct current power distribution network with shorter line distance, so that the accuracy requirement is difficult to meet. According to the method, an LC injection device is connected in series to a system under the condition of system power loss, parameters are adjusted to enable a circuit to meet the condition of under-damped oscillation, and nonlinear least square fitting is utilized to fit oscillating current to obtain oscillation frequency and attenuation coefficient so as to obtain fault distance. For this reason, documents [1] - - [ xuming, xiao li shi, wang hai feng, lin liang zhen ], a direct current distribution network cable fault location method based on the Prony algorithm [ J ]. a new electrical energy technology for electricians, 2015,34(04):1-5+30 ], propose to eliminate the influence caused by non-uniform inductance by using a double-end measurement method, and extract the oscillation frequency and the attenuation coefficient of the discharge current by applying the first-order decomposition of the Prony algorithm. The method needs to know the unit resistance and the total length of the line in advance, although the total length of the line is basically not changed, the unit resistance of the line is changed under the influence of the surrounding environment and the skin effect, and in addition, the method has larger ranging error when the distributed capacitance of the line is considered in the actual ranging.
disclosure of Invention
aiming at the defects in the background art, the invention provides a direct-current power distribution network double-end ranging fault positioning method based on an improved injection method, and solves the technical problem that the existing ranging method is large in ranging error.
The technical scheme of the invention is realized as follows:
A direct-current distribution network double-end ranging fault locating method based on an improved injection method is characterized in that fault ranging modules are arranged at two ends of a direct-current distribution network, and each fault ranging module comprises a capacitor, an inductor, a first single-pole triple-throw switch and a second single-pole triple-throw switch; the output end of the first single-pole three-throw switch is respectively connected with the ends a, b and c, the end a is connected with the positive electrode of a direct-current power supply, the end b is connected with the positive electrode of a direct-current power distribution network, the end c is grounded, the input end of the first single-pole three-throw switch is connected with an inductor, the inductor is connected with a capacitor, the capacitor is connected with the input end of a second single-pole three-throw switch, the output end of the second single-pole three-throw switch is respectively connected with the ends d, e and f, the end d is connected with the negative electrode of the direct-current power supply, the end e is connected with the negative electrode of the direct-current power distribution;
When the direct-current power distribution network at the two ends normally operates, the circuit breaker is closed, the first single-pole triple-throw switch is connected with the end a, the second single-pole triple-throw switch is connected with the end d, and the direct-current power supply is connected with the capacitor to charge the capacitor; when a short-circuit fault occurs, the first single-pole triple-throw switch is disconnected with the end a, the second single-pole triple-throw switch is disconnected with the end d, the fault location module is connected into a fault line, and fault location is carried out according to the fault location step.
the fault location module comprises a fault location unit I and a fault location unit II, the fault location unit I is arranged at the end I of the direct-current power distribution network, and the fault location unit II is arranged at the end II of the direct-current circuit;
the fault distance measuring unit I comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S1 and a second single-pole three-throw switch S2, wherein the output end of the first single-pole three-throw switch S1 is connected with ends a, b and C respectively, the end a is connected with the positive pole of a direct-current power supply, the end b is connected with the positive pole of the direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S1 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S2, the output end of the second single-pole three-throw switch S2 is connected with ends d, e and f respectively, the end d is connected with the negative pole of the direct-current power supply, the end e is connected with the negative pole of the direct-;
the fault distance measuring unit II comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S3 and a second single-pole three-throw switch S4, wherein the output end of the first single-pole three-throw switch S3 is connected with ends a, b and C respectively, the end a is connected with the positive pole of a direct-current power supply, the end b is connected with the positive pole of the direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S3 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S4, the output end of the second single-pole three-throw switch S4 is connected with ends d, e and f respectively, the end d is connected with the negative pole of the direct-current power supply, the end e is connected with the negative pole of the direct-.
when a single-pole short-circuit fault occurs, the fault distance measurement step is as follows:
s1, enabling the end II of the direct current power distribution network to normally operate, enabling the end I of the direct current power distribution network to be connected to a fault distance measuring unit I to form a fault circuit I, enabling the fault circuit I to be equivalent to an RL equivalent circuit, and calculating the distance between a fault point and the fault distance measuring unit I at the end I of the direct current power distribution network;
s2, the end I of the direct current power distribution network normally runs, the end II of the direct current power distribution network is connected to a fault distance measuring unit II to form a fault circuit II, the fault circuit II is equivalent to an RL equivalent circuit, and the distance between a fault point and the fault distance measuring unit II of the end II of the direct current power distribution network is calculated;
And S3, combining the distance in the step S1 and the distance in the step S2 to eliminate inductance parameters of the direct current distribution network, and obtaining the relation between the distance between the fault point and the fault distance measurement unit I at the end I of the direct current distribution network and the total length of the direct current distribution network.
The method for calculating the distance from the fault point to the fault distance measurement unit I at the end I of the direct current distribution network in the step S1 comprises the following steps:
S1.1, a first single-pole three-throw switch S1 of a fault location unit I is connected with a b end, a second single-pole three-throw switch S2 is connected with an f end, a discharge loop is formed at the I end of a direct-current power distribution network, and a fault circuit is equivalent to an RL equivalent circuit;
S1.2, writing a loop equation according to the voltage-current relation of the inductor and the capacitor, and calculating a second-order differential equation related to the discharge current:
Wherein I (t) is the current of the I-end loop of the direct-current power distribution network, Ra and La are the total resistance and the total inductance of the I-end current loop of the direct-current power distribution network respectively, and C is the capacitance of the fault location unit;
S1.3, inductance and capacitance parameters in the fault location unit I are adjusted to enable the RL equivalent circuit to meet the under-damped oscillation condition, and the current I (t) of the I end loop of the direct-current power distribution network is as follows:
Wherein K1 is a coefficient, ω d1 is a damping oscillation frequency of the I-end current loop of the direct-current distribution network, and α 1 is an attenuation constant of the I-end current loop of the direct-current distribution network;
S1.4, calculating a damping oscillation frequency omega d1, an attenuation constant alpha 1 and a coefficient K1 by utilizing nonlinear least square fitting;
s1.5, according to the relationship between the resonant angular frequency ω n1 and the damped oscillation frequency ω d1 and the relationship between the resonant angular frequency ω n1 and the fault distance x, we can obtain:
ω＝ω+α (3)，
Wherein Lp is inductance of the fault distance measuring unit, l0 is inductance of the unit length of the direct-current power distribution network, and x is the distance between a fault point and the fault distance measuring unit I at the end I of the direct-current power distribution network;
S1.6, the fault distance x can be obtained according to the formula (3) and the formula (4) as follows:
the method for calculating the distance between the fault point and the fault distance measurement unit II at the end II of the direct current power distribution network comprises the following steps: the first single-pole three-throw switch S3 of the fault distance measuring unit II is connected with the b end, the second single-pole three-throw switch S4 is connected with the f end, the direct-current distribution network II end forms a discharging loop, and the distance between a fault point and the fault distance measuring unit II of the direct-current distribution network II end can be obtained according to the steps S1.2 to S1.5:
Wherein, L is the total line length of the direct current distribution network, alpha 2 is the attenuation constant of the current loop at the II end of the direct current distribution network, and omega d2 is the oscillation frequency of the current loop at the II end of the direct current distribution network.
the relation between the distance from the fault point to a fault distance measuring unit I at the end I of the direct-current power distribution network and the total length of the direct-current power distribution network is as follows:
When a double-pole short-circuit fault occurs, both the end I and the end II of the direct-current power distribution network are connected to a fault distance measurement module, a first single-pole three-throw switch S1 of a fault distance measurement unit I is connected with the end b, a second single-pole three-throw switch S2 is connected with the end f, the end I of the direct-current power distribution network forms a discharge loop, a first single-pole three-throw switch S3 of the fault distance measurement unit II is connected with the end b, a second single-pole three-throw switch S4 is connected with the end f, the end II of the direct-current power distribution network forms a discharge loop, and the distance x from a fault point to the fault distance measurement unit I of the end I of the direct-current power distribution network and the distance L-x from the fault point to the fault distance measurement unit II of the end:
by combining equation (8) and equation (9), the fault distance x can be obtained:
and the inductance Lp and the capacitance C in the fault distance measuring unit I and the fault distance measuring unit II meet the second-order circuit damping condition:
wherein Rf is transition resistance.
The beneficial effect that this technical scheme can produce:
1) the invention enables the fault location module and the fault line to form an under-damped RLC circuit, obtains the oscillation frequency and the attenuation coefficient by utilizing nonlinear least square fitting, replaces the line resistance and the inductance in the traditional injection method by the total line length which is basically unchanged when the fault distance is measured by introducing double-end information quantity, and has simple principle and higher positioning precision.
2) The invention is suitable for the condition that the transition resistance in the urban power distribution network is small, the distance measurement module can repeatedly measure, the accidental condition is avoided, the reliability and the accuracy of the distance measurement are greatly improved, and the invention has certain application value.
Drawings
in order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic structural diagram of a fault location module according to the present invention.
Fig. 2 is a schematic diagram of the structure of the left-side current path in the case of a single-stage ground fault.
Fig. 3 is an equivalent circuit at the time of a single-stage ground fault.
Fig. 4 is a waveform of the capacitor discharge current of fig. 3.
fig. 5 is a schematic diagram of the structure of the current path in the event of a double short-circuit fault.
Fig. 6 is an equivalent circuit at the time of bipolar short-circuit failure.
Fig. 7 is a discharge current waveform of the 1-terminal fault ranging module.
fig. 8 is a discharge current waveform of the 2-terminal fault ranging module.
Fig. 9 is a fitting result of the 1-terminal fault ranging module discharge current.
Fig. 10 is a fitting result of the 2-terminal fault ranging module discharge current.
Fig. 11 is a discharge current waveform of the 1-terminal fault ranging module.
Fig. 12 is a discharge current waveform of the 2-terminal fault ranging module.
Fig. 13 is a fitting result of the 1-terminal fault ranging module discharge current.
fig. 14 is a fitting result of the 2-terminal fault ranging module discharge current.
Detailed Description
the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive effort based on the embodiments of the present invention, are within the scope of the present invention.
The topological structure of the direct-current power distribution network is mainly divided into three types, namely a radiation type, a two-end power supply type and a ring type. At present, a bipolar operation mode and a two-end power supply type topological structure are mainly adopted. Therefore, the invention takes the structure of the two-end power supply type direct current distribution network with the voltage of the direct current power supply being +/-10 kV and the line length L of the direct current distribution network being 10km under the bipolar operation mode as a research object, and the fault location module is shown as figure 1.
a direct-current distribution network double-end ranging fault locating method based on an improved injection method is characterized in that fault ranging modules are arranged at two ends of a direct-current distribution network, and each fault ranging module comprises a capacitor, an inductor, a first single-pole triple-throw switch and a second single-pole triple-throw switch; the output end of the first single-pole three-throw switch is respectively connected with the ends a, b and c, the end a is connected with the positive electrode of a direct-current power supply, the end b is connected with the positive electrode of a direct-current power distribution network, the end c is grounded, the input end of the first single-pole three-throw switch is connected with an inductor, the inductor is connected with a capacitor, the capacitor is connected with the input end of a second single-pole three-throw switch, the output end of the second single-pole three-throw switch is respectively connected with the ends d, e and f, the end d is connected with the negative electrode of the direct-current power supply, the end e is connected with the negative electrode of the direct-current power distribution.
when the direct-current power distribution network at two ends normally operates, the circuit breaker is closed, the first single-pole three-throw switch is connected with the anode a end, the second single-pole three-throw switch is connected with the cathode d end, and the direct-current power supply is connected with the capacitor to charge the capacitor; when a short-circuit fault occurs, the first single-pole triple-throw switch is disconnected with the end a, the second single-pole triple-throw switch is disconnected with the end d, the fault location module is connected into a fault line, and fault location is carried out according to the fault location step.
the fault location module comprises a fault location unit I and a fault location unit II, the fault location unit I is arranged at the end of the direct current power distribution network I, and the fault location unit II is arranged at the end of the direct current power distribution network II. The fault distance measuring unit I comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S1 and a second single-pole three-throw switch S2, a first single-pole three-throw switch S1 and a second single-pole three-throw switch S2, wherein the output end of the first single-pole three-throw switch S1 is respectively connected with ends a, b and C, the end a is connected with the positive electrode of a direct-current power supply, the end b is connected with the positive electrode of a direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S1 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S2, the output end of the second single-pole three-throw switch S2 is respectively connected with ends d, e and f, the end d is connected with the negative electrode of the direct-current power supply, the end e is connected with the negative electrode; the fault distance measuring unit II comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S3 and a second single-pole three-throw switch S4, wherein the output end of the first single-pole three-throw switch S3 is connected with ends a, b and C respectively, the end a is connected with the positive pole of a direct-current power supply, the end b is connected with the positive pole of the direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S3 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S4, the output end of the second single-pole three-throw switch S4 is connected with ends d, e and f respectively, the end d is connected with the negative pole of the direct-current power supply, the end e is connected with the negative pole of the direct-.
When a single-pole short-circuit fault occurs, the fault distance measurement step is as follows:
when the anode of the I end of the direct-current power distribution network has a single-pole short-circuit fault, the first single-pole three-throw switch S1 is disconnected with the a end, the second single-pole three-throw switch S2 is disconnected with the d end, the first single-pole three-throw switch S1 of the I end of the direct-current power distribution network is connected with the b end after 0.5S delay, the second single-pole three-throw switch S2 is connected with the f end, and the fault distance measuring unit I is connected into a fault line. The delay of 0.5s is to avoid the transient current generated when the breaker is opened from affecting the fault ranging process. And after 1S, the first single-pole-three-throw switch S1 and the second single-pole-three-throw switch S2 are restored and are connected with the direct-current power supply again to charge the capacitor, so that preparation is made for next fault location. And after 1.5s, the fault location unit II at the end II of the direct current power distribution network is accessed into a fault line, and the operation method is the same as that of the fault location unit I at the end I of the direct current power distribution network when the fault location unit II is accessed. The time difference of the access of the fault location modules at the two ends is 0.5s, so that the current of the fault location module at one end can be prevented from influencing the fault location module at the other end.
and S1, the end II of the direct current distribution network normally runs, the end I of the direct current distribution network is connected with the fault distance measuring unit I to form a fault circuit I, the fault circuit I is equivalent to an RL equivalent circuit, and the distance between a fault point and the fault distance measuring unit I at the end I of the direct current distribution network is calculated.
the method for calculating the distance between the fault point and the fault distance measurement unit I comprises the following steps:
S1.1, a first single-pole three-throw switch S1 of a fault location unit I is connected with a b end, a second single-pole three-throw switch S2 is connected with an f end, a discharge loop is formed at the I end of a direct-current power distribution network, and a fault circuit is equivalent to an RL equivalent circuit. The discharge loop and RL equivalent circuit for a single pole short fault are shown in fig. 2 and 3, respectively.
S1.2, writing a loop equation according to the voltage-current relation of the inductor and the capacitor, and calculating a second-order differential equation related to the discharge current:
wherein I (t) is the current of the I-end loop of the direct-current power distribution network, Ra and La are the total resistance and the total inductance of the I-end current loop of the direct-current power distribution network respectively, and C is the capacitance of the fault location unit.
S1.3, inductance and capacitance parameters in the fault location unit I are adjusted to enable the RL equivalent circuit to meet the under-damped oscillation condition, and the current I (t) of the I end loop of the direct-current power distribution network is as follows:
k1 is a coefficient, omega d1 is the damping oscillation frequency of the I-end current loop of the direct-current distribution network, and alpha 1 is the attenuation constant of the I-end current loop of the direct-current distribution network. The discharge current waveform is shown in fig. 4.
s1.4, calculating the damped oscillation frequency omega d1, the attenuation constant alpha 1 and the coefficient K1 by utilizing nonlinear least square fitting.
s1.5, according to the relationship between the resonant angular frequency ω n1 and the damped oscillation frequency ω d1 and the relationship between the resonant angular frequency ω n1 and the fault distance x, we can obtain:
ω＝ω+α (3)，
In the formula (4), Lp is inductance of the ranging module, l0 is inductance per unit length of the direct current power distribution network, and x is the distance between a fault point and a fault ranging unit I at the end I of the direct current power distribution network.
s1.6, the fault distance x can be obtained according to the formula (3) and the formula (4) as follows:
and S2, the end I of the direct current distribution network normally runs, the end II of the direct current distribution network is connected to the fault distance measuring unit II to form a fault circuit II, the fault circuit II is equivalent to an RL equivalent circuit, and the distance between a fault point and the fault distance measuring unit II of the end II of the direct current distribution network is calculated. The method for calculating the distance between the fault point and the fault distance measurement unit II comprises the following steps: the first single-pole three-throw switch S3 of the fault distance measuring unit II is connected with the b end, the second single-pole three-throw switch S4 is connected with the f end, the direct-current distribution network II end forms a discharging loop, and the distance between a fault point and the fault distance measuring unit II of the direct-current distribution network II end can be obtained according to the steps S1.2 to S1.5:
Wherein, L is the total line length of the direct current distribution network, alpha 2 is the attenuation constant of the current loop at the II end of the direct current distribution network, and omega d2 is the oscillation frequency of the current loop at the II end of the direct current distribution network.
S3, eliminating an inductance parameter l0 of the direct current distribution network by combining the distance in the step S1 and the distance in the step S2, and obtaining the relation between the distance between the fault point and the fault distance measurement unit I at the end I of the direct current distribution network and the total length of the direct current distribution network as follows:
When a double-pole short-circuit fault occurs, the end I of the direct-current power distribution network and the end II of the direct-current power distribution network are respectively connected to the fault distance measurement module, the first single-pole three-throw switch S1 of the fault distance measurement unit I is connected with the end b, the second single-pole three-throw switch S2 is connected with the end f, the end I of the direct-current power distribution network forms a discharge loop, the first single-pole three-throw switch S3 of the fault distance measurement unit II is connected with the end b, the second single-pole three-throw switch S4 is connected with the end f, the end II of the direct-current power distribution network forms a discharge loop, and the discharge loop and an equivalent. According to the steps S1 and S2, the distance x between the fault point and the fault distance measuring unit I at the end I of the direct current distribution network and the distance L-x between the fault point and the fault distance measuring unit II at the end II of the direct current distribution network can be obtained:
By combining equation (8) and equation (9), the fault distance x can be obtained:
Simulation analysis:
A simulation model of a two-end power supply type direct-current power distribution network and a distance measurement module is built in MATLAB/SIMULINK, wherein the voltage of a direct-current power supply is +/-10 kV and the line length L of the direct-current power distribution network is 10km in a bipolar operation mode shown in figure 1. Since the measurement by the injection method occurs after the circuit breaker is opened, data of the converter station and the alternating current side are not needed during the measurement, and only the parameters of the direct current line are considered during the parameter setting. The line and ranging module parameter settings are shown in table 1.
TABLE 1 line and ranging module parameters
setting the inductance and the capacitance in the fault location unit I and the fault location unit II to meet the second-order circuit damping condition:
wherein, Rf is transition resistance, namely the grounding resistance of the unipolar fault and the resistance at the connection part of the two polar lines of the bipolar fault. The smaller the left part of the inequality (11) is, the larger the right part is, and the easier the under-damping condition is satisfied. As for the left side of the inequality (11), when a fault occurs at the end of the line, the resistance of the fault line is the total length of the line, and at this time, the value of r0x on the left side of the inequality (11) is the largest, and as can be seen from table 1, the total resistance Ra of the current loop is 1.2 Ω. Under the condition that high-resistance grounding faults are not considered (a high-resistance grounding phenomenon in the power distribution network is few and special research on a high-resistance grounding fault positioning method is available), the grounding faults of the urban power distribution network are metallic and small transition resistance grounding faults generally, and the transition resistance is generally smaller than 16 omega, so that the left side of the inequality (11) is smaller than 20 omega; for the right side of the inequality (11), when a fault occurs at the initial end of the line, l0x can be ignored, the value of the right side of the inequality (11) is the minimum, the inductance value of the fault distance measuring module is set to be 100 times of the capacitance, therefore, the size of the right side is 20 omega, the inequality (11) is established, and when the fault does not occur at the initial end of the line, the right side is larger due to the existence of l0x, and the inequality (11) is established. In order to improve the distance measurement precision, the inductance of the fault distance measurement module and the unit inductance of the direct-current distribution network are set to be in the same order of magnitude, and one percent of the inductance is taken as the capacitance of the fault distance measurement module. The capacitance and inductance settings of the fault location module are shown in table 1.
Because the ranging principle of the unipolar ground fault and the bipolar short-circuit fault is basically the same, simulation analysis is performed by taking the unipolar ground fault as an example. In the deduction process of the fault positioning algorithm, the influence of the distributed capacitance of the line is ignored to simplify a circuit model, and the uncertainty of the transition resistance Rf in the fault occurrence is considered, so that the method verifies three conditions of not considering the distributed capacitance, considering the distributed capacitance and the change of the transition resistance, performs comparative analysis with other methods, and the result proves the effectiveness of the method.
(1) fault location without consideration of distributed capacitance
when the distributed capacitance of the direct current line is not considered, assuming that a single-pole ground fault occurs, the actual distance d between a fault point and a fault distance measurement unit I at the end I of the direct current distribution network is 2km, and the transition resistance is 4 omega, and obtaining sine waves with smooth oscillation attenuation of the discharge current waveforms of fault distance measurement modules at two ends which are successively input, as shown in fig. 7 and 8 respectively.
After the discharge current data of the loops at the two ends are obtained through the fault distance measuring modules at the two ends, nonlinear least square fitting is carried out on the discharge current data to obtain that the attenuation coefficients alpha 1 and alpha 2 are 251.31 and 174.19 respectively, and the damping oscillation frequencies omega d1 and omega d2 are 2079.69rad/s and 1602.16rad/s respectively. As shown in fig. 9 and 10, the measured discharge current substantially coincides with the fitted discharge current, and the fitting effect is good.
substituting the attenuation coefficients alpha 1 and alpha 2 and the damping oscillation frequencies omega d1 and omega d2 into the formula (7) can obtain that the distance between the fault point and the fault location module at the I end of the direct current distribution network is 2.0001km, and the fault location error percentage is 0.05%. Because the distributed capacitance of the line is ignored in the simulation, the simulation is an ideal condition which does not exist in practice, and therefore, errors are basically avoided.
(2) Fault location taking into account distributed capacitance
Distributed capacitance exists in an actual power transmission line, and at the moment, an equivalent model of the line is a pi-type equivalent circuit. Still assume that a single-pole ground fault occurs, the actual distance d between the fault point and the distance measurement module at the I end of the direct-current distribution network is 2km, the transition resistance is 4 omega, and the distributed capacitance value is 0.35 muF/km, which is listed in Table 1. At this time, the waveforms of the discharge currents of the two-terminal fault location modules that are sequentially turned on are shown in fig. 11 and 12, respectively.
as can be seen from fig. 11 and 12, the discharge current waveform is no longer a smooth oscillation attenuation curve due to the influence of the distributed capacitance, and the discharge current data therein is fitted by using nonlinear least square fitting to obtain attenuation coefficients α 1 and α 2 of 252.81 and 179.31, respectively, and damped oscillation frequencies ω d1 and ω d2 of 2080.27rad/s and 1592.38rad/s, respectively. As shown in fig. 13 and 14, the measured discharge current substantially agrees with the fitted trend.
substituting the attenuation coefficients alpha 1 and alpha 2 and the damping oscillation frequencies omega d1 and omega d2 into the formula (7), obtaining that the distance x between the fault point and the distance measurement module at the I end of the direct current distribution network is 1.9618km, and the distance measurement error percentage epsilon is as follows:
The ranging error percentage meets the accuracy requirement of fault location. In order to further verify the effectiveness of the proposed fault location method, simulation experiments are carried out on different fault distances, and corresponding distance measurement errors are listed in table 2, wherein the actual fault distance and the fault distance measurement distance both represent the distance from a fault point to a fault distance measurement module at the I end of the direct current power distribution network.
TABLE 2 error percentages for fault ranging at different distances
As can be seen from Table 2, for the DC line model considering the distributed capacitance, the method of the invention still has higher ranging accuracy, and for the 10km transmission line, the error percentage is less than 0.4%, which meets the accuracy requirement of fault location.
(3) Fault location taking into account distributed capacitance and transition resistance changes
In actual operation, the size of the transition resistor is uncertain, so that the influence of the change of the size of the transition resistor on the distance measurement precision needs to be researched. Assuming that a 10km line has a single-pole ground fault and the distance between a fault point and a fault location module at the I end of the direct-current power distribution network is 2km, considering the influence of distributed capacitance, and listing the location distances and error percentages when different transition resistances exist at the same fault distance in table 3.
TABLE 3 ranging error percentage at different transition resistances for the same fault distance
as can be seen from table 3, the ranging error percentage gradually increases with increasing transition resistance, but is always less than 0.5%. Because the transition resistance of the urban direct current distribution network line is generally metallic and small, and the numerical value is not large, when the transition resistance is a large value of 16 omega, the ranging error percentage at the fault distance of 2km is 0.484%, the error percentage is maximum, and the ranging precision is high at the moment and still meets the ranging precision requirement. Thus, the method of the invention has a certain ability to withstand transition resistance.
(4) Distance measurement method comparison
the method of the present invention is compared with the method of document [1] in consideration of the distributed capacitance. The actual distance between a fault point and a fault distance measurement module at the I end of the direct current distribution network is 2km and the actual distance between the fault point and the fault distance measurement module at the I end of the direct current distribution network is 4 omega, and the parameter values are set as shown in table 1.
TABLE 4 two methods Fault location result comparison
As can be seen from table 4, the method proposed in the document [1] has a large error of 7.532% at most when the distributed capacitance is considered, and cannot meet the requirement of positioning accuracy, and further, because the fault distance expression of the document [1] has a resistance parameter, the distance measurement error will be further increased when the resistance is changed by various factors, whereas the method of the present invention has a high distance measurement accuracy when the distributed capacitance is considered, and for a 10km power transmission line, the error percentage is less than 0.4%, and further, the method of the present invention eliminates the influence of the line inductance and the resistance parameter on the distance measurement result, and the positioning accuracy is higher.
the above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (8)
1. A double-end distance measurement fault location method for a direct-current power distribution network based on an improved injection method is characterized in that fault location modules are arranged at two ends of the direct-current power distribution network, and each fault location module comprises a capacitor, an inductor, a first single-pole three-throw switch and a second single-pole three-throw switch; the output end of the first single-pole three-throw switch is respectively connected with the ends a, b and c, the end a is connected with the positive electrode of a direct-current power supply, the end b is connected with the positive electrode of a direct-current power distribution network, the end c is grounded, the input end of the first single-pole three-throw switch is connected with an inductor, the inductor is connected with a capacitor, the capacitor is connected with the input end of a second single-pole three-throw switch, the output end of the second single-pole three-throw switch is respectively connected with the ends d, e and f, the end d is connected with the negative electrode of the direct-current power supply, the end e is connected with the negative electrode of the direct-current power distribution;
when the direct-current power distribution network at the two ends normally operates, the circuit breaker is closed, the first single-pole triple-throw switch is connected with the end a, the second single-pole triple-throw switch is connected with the end d, and the direct-current power supply is connected with the capacitor to charge the capacitor; when a short-circuit fault occurs, the first single-pole triple-throw switch is disconnected with the end a, the second single-pole triple-throw switch is disconnected with the end d, the fault location module is connected into a fault line, and fault location is carried out according to the fault location step.
2. the direct-current power distribution network double-end ranging fault locating method based on the improved injection method as claimed in claim 1, wherein the fault ranging module comprises a fault ranging unit I and a fault ranging unit II, the fault ranging unit I is arranged at the end of a direct-current power distribution network I, and the fault ranging unit II is arranged at the end of a direct-current line II;
The fault distance measuring unit I comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S1 and a second single-pole three-throw switch S2, wherein the output end of the first single-pole three-throw switch S1 is connected with ends a, b and C respectively, the end a is connected with the positive pole of a direct-current power supply, the end b is connected with the positive pole of the direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S1 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S2, the output end of the second single-pole three-throw switch S2 is connected with ends d, e and f respectively, the end d is connected with the negative pole of the direct-current power supply, the end e is connected with the negative pole of the direct-;
The fault distance measuring unit II comprises a capacitor C, an inductor Lp, a first single-pole three-throw switch S3 and a second single-pole three-throw switch S4, wherein the output end of the first single-pole three-throw switch S3 is connected with ends a, b and C respectively, the end a is connected with the positive pole of a direct-current power supply, the end b is connected with the positive pole of the direct-current power distribution network, the end C is grounded, the input end of the first single-pole three-throw switch S3 is connected with the inductor Lp, the inductor Lp is connected with the capacitor C, the capacitor C is connected with the input end of the second single-pole three-throw switch S4, the output end of the second single-pole three-throw switch S4 is connected with ends d, e and f respectively, the end d is connected with the negative pole of the direct-current power supply, the end e is connected with the negative pole of the direct-.
3. the double-end ranging fault locating method for the direct-current power distribution network based on the improved injection method as claimed in claim 1 or 2, wherein when a single-pole short-circuit fault occurs, the fault ranging step is as follows:
S1, enabling the end II of the direct current power distribution network to normally operate, enabling the end I of the direct current power distribution network to be connected to a fault distance measuring unit I to form a fault circuit I, enabling the fault circuit I to be equivalent to an RL equivalent circuit, and calculating the distance between a fault point and the fault distance measuring unit I at the end I of the direct current power distribution network;
s2, the end I of the direct current power distribution network normally runs, the end II of the direct current power distribution network is connected to a fault distance measuring unit II to form a fault circuit II, the fault circuit II is equivalent to an RL equivalent circuit, and the distance between a fault point and the fault distance measuring unit II of the end II of the direct current power distribution network is calculated;
And S3, combining the distance in the step S1 and the distance in the step S2 to eliminate inductance parameters of the direct current distribution network, and obtaining the relation between the distance between the fault point and the fault distance measurement unit I at the end I of the direct current distribution network and the total length of the direct current distribution network.
4. The double-end ranging fault location method for the direct current distribution network based on the improved injection method as claimed in claim 3, wherein the calculation method of the distance from the fault point to the fault ranging unit I at the end I of the direct current distribution network in the step S1 is as follows:
S1.1, a first single-pole three-throw switch S1 of a fault location unit I is connected with a b end, a second single-pole three-throw switch S2 is connected with an f end, a discharge loop is formed at the I end of a direct-current power distribution network, and a fault circuit is equivalent to an RL equivalent circuit;
S1.2, writing a loop equation according to the voltage-current relation of the inductor and the capacitor, and calculating a second-order differential equation related to the discharge current:
wherein I (t) is the current of the I-end loop of the direct-current power distribution network, Ra and La are the total resistance and the total inductance of the I-end current loop of the direct-current power distribution network respectively, and C is the capacitance of the fault location unit;
S1.3, inductance and capacitance parameters in the fault location unit I are adjusted to enable the RL equivalent circuit to meet the under-damped oscillation condition, and the current I (t) of the I end loop of the direct-current power distribution network is as follows:
Wherein K1 is a coefficient, ω d1 is a damping oscillation frequency of the I-end current loop of the direct-current distribution network, and α 1 is an attenuation constant of the I-end current loop of the direct-current distribution network;
S1.4, calculating a damping oscillation frequency omega d1, an attenuation constant alpha 1 and a coefficient K1 by utilizing nonlinear least square fitting;
s1.5, according to the relationship between the resonant angular frequency ω n1 and the damped oscillation frequency ω d1 and the relationship between the resonant angular frequency ω n1 and the fault distance x, we can obtain:
ω＝ω+α (3)，
Wherein Lp is inductance of the fault distance measuring unit, l0 is inductance of the unit length of the direct-current power distribution network, and x is the distance between a fault point and the fault distance measuring unit I at the end I of the direct-current power distribution network;
s1.6, the fault distance x can be obtained according to the formula (3) and the formula (4) as follows:
5. The direct current distribution network double-end ranging fault locating method based on the improved injection method as claimed in claim 4, wherein the calculation method of the distance from the fault point to the fault ranging unit II at the end II of the direct current distribution network is as follows: the first single-pole three-throw switch S3 of the fault distance measuring unit II is connected with the b end, the second single-pole three-throw switch S4 is connected with the f end, the direct-current distribution network II end forms a discharging loop, and the distance between a fault point and the fault distance measuring unit II of the direct-current distribution network II end can be obtained according to the steps S1.2 to S1.5:
Wherein, L is the total line length of the direct current distribution network, alpha 2 is the attenuation constant of the current loop at the II end of the direct current distribution network, and omega d2 is the oscillation frequency of the current loop at the II end of the direct current distribution network.
6. The direct current distribution network double-end ranging fault locating method based on the improved injection method as claimed in claim 5, wherein the relation between the distance between the fault point and the fault ranging unit I at the end I of the direct current distribution network and the total length of the direct current distribution network is as follows:
7. The double-end fault location method based on the improved injection method for the direct-current distribution network according to claim 1 or 6, wherein when a double-pole short-circuit fault occurs, both the end I and the end II of the direct-current distribution network are connected to the fault location module, the first single-pole three-throw switch S1 of the fault location unit I is connected to the end b, the second single-pole three-throw switch S2 is connected to the end f, the end I of the direct-current distribution network forms a discharge loop, the first single-pole three-throw switch S3 of the fault location unit II is connected to the end b, the second single-pole three-throw switch S4 is connected to the end f, the end II of the direct-current distribution network forms a discharge loop, and the distance x from the fault point to the fault location unit I of the end I of the direct-current distribution network and the distance L-x from the fault point to the fault location unit II of the end of the direct-current distribution network:
By combining equation (8) and equation (9), the fault distance x can be obtained:
8. The direct-current distribution network double-end ranging fault locating method based on the improved injection method as claimed in claim 7, wherein the inductance Lp and the capacitance C in the fault ranging unit I and the fault ranging unit II meet a second-order circuit damping condition:
Wherein Rf is transition resistance.
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