CN114814463A

CN114814463A - Single-ended direct current distribution line single-pole ground fault location method

Info

Publication number: CN114814463A
Application number: CN202210511978.9A
Authority: CN
Inventors: 刘昊; 岳德臣; 陶刚; 陶杰
Original assignee: China University of Mining and Technology CUMT
Current assignee: China University of Mining and Technology CUMT
Priority date: 2022-05-12
Filing date: 2022-05-12
Publication date: 2022-07-29

Abstract

The invention discloses a single-ended direct current distribution line single-pole ground fault distance measurement method, and belongs to the technical field of high-voltage direct current power distribution. The purpose is when taking place monopole ground connection short circuit to load place feeder, can be fast and accurate location and range finding. The main scheme is that the head ends of the positive feeder line and the negative feeder line are respectively provided with a distance measurement module for measuring the distance of the fault feeder line, the least square fitting is carried out on current waveforms respectively measured by the two distance measurement modules to obtain respective corresponding parameters, and the parameters are substituted into a theoretical fault measurement distance formula for calculation. The method overcomes the defects that the traditional distance measurement method is inaccurate in measurement result and has no compatibility with a longer line, improves the measurement accuracy, can accurately position the fault within a range of 40m, and has remarkable superiority.

Description

Single-ended direct current distribution line single-pole ground fault location method

Technical Field

The invention belongs to the technical field of high-voltage direct-current power distribution, and particularly relates to a single-ended direct-current distribution line single-pole ground fault distance measurement method. The method is mainly suitable for fast and accurate positioning and ranging when the single-pole grounding short circuit occurs to the feeder line where the load is located.

Background

The high-voltage direct-current power distribution has the characteristics of long transmission distance, low line manufacturing cost, high monopole transmission power, good control performance and the like, and is an important means for solving high-voltage, high-capacity, long-distance power transmission and asynchronous networking in all developed countries at present. With the rapid development of the Chinese power industry, the transmission capacitance is further expanded, the connection among systems is more and more compact, and the direct current power distribution project plays an important role in the construction of the Chinese power grid.

The direct current power transmission and distribution technology in China is still in a development stage, and how to realize rapid fault detection and fault location is one of key technical problems limiting the development of the direct current power transmission and distribution technology. Data indicate that most faults occur in a dc power distribution system, generally on the line, and particularly single-pole ground faults and inter-pole short-circuit faults. The direct current distribution line comprises transient faults and permanent faults, the transient faults can be processed by means of a relay protection device, and once the permanent faults occur, the power system operation maintenance personnel are required to reach a fault point to check the problems and solve the faults, so that power supply can be restored. The direct current lines are mostly used as interconnection lines among all power grids or undertake ultrahigh-power distribution tasks, faults of the direct current lines have great influence on the operation of a power system and industrial and agricultural production, and the great development of the direct current distribution line fault location technology has great significance. The former distribution lines all break down and need artifical patrolling the plumber to inspect and get rid of, because the environment in a lot of distribution corridors is abominable, the topography is complicated, has brought huge degree of difficulty for patrolling the line. Under the condition that a patrol worker cannot find a fault position quickly, the novel direct-current distribution line fault location technology becomes a popular topic for research in power system departments at home and abroad.

With the large access of new energy, the direct current transmission and distribution system can reduce the use of the converter equipment,

compared with the use of an alternating current device, the device has higher practicability and economy. In recent years, fully-controlled power electronic equipment develops rapidly, and particularly, a flexible Direct-Current power distribution technology (Modular Multilevel Converter, High Voltage Direct Current (MMC) HVDC) formed by Modular Multilevel converters can rapidly and independently adjust active power and reactive power, and is convenient for distributed power supply grid connection and the like. However, when the flexible dc distribution line fails, not only the normal transmission of power is affected, but also the converter equipment is damaged, and even the system is shut down, and it is important to quickly detect the failure of the flexible dc distribution line. The fault detection isolation technology and the fault location technology are key technologies for flexible direct power distribution development, and accurate location needs to be completed by means of the fault location technology, so that a line is repaired in time, and normal operation of a system is restored.

Disclosure of Invention

The invention aims to provide a single-ended direct-current distribution line single-pole ground fault location method, which solves the problem that the fault distance is difficult to accurately locate when a feeder line where a load is located fails in the traditional injection method. The method overcomes the defect of single injection method distance measurement, and is successfully applied to the single-ended direct current distribution network feeder line fault distance measurement, so that the measurement accuracy is remarkably improved.

The invention is realized by the following technical scheme: a single-ended direct current distribution line single-pole ground fault distance measurement method is characterized in that a distance measurement module is respectively arranged at the head ends of a positive feeder line and a negative feeder line to measure the distance of a fault feeder line, the current waveforms respectively measured by the two distance measurement modules are used for carrying out least square fitting to obtain respective corresponding parameters, and the parameters are brought into a theoretical fault measurement distance formula to obtain a theoretical fault distance value.

Furthermore, the distance measurement module consists of an inductor, a capacitor, a direct current power supply and a switch, the distance measurement module and a fault line form a second-order discharge circuit of underdamped oscillation by adjusting the sizes of the inductor and the capacitor, the oscillation frequency and the attenuation coefficient of discharge current attenuation are determined by utilizing nonlinear least square fitting, a fault distance measurement algorithm in the traditional injection method is improved by introducing double-end information, a fault distance calculation formula with the total length of the line and the inductance and capacitance of the distance measurement module as parameters is obtained, and the influence of the inductor and the resistance of the direct current line on fault positioning is eliminated.

Fault distance obtained by solving in series

The calculation formula of (2) is as follows:

the parameters obtained by simultaneous double-end measurement can eliminate the parameters of line inductance and resistance, and the fault distance expression expressed by the total line length L is obtained as follows:

in the formula:

the total length of a direct current distribution network line;

and

respectively the damped oscillation frequency and the damping constant of the loop.

Compared with the prior art, the method has the advantages that the defects that the traditional distance measurement method is inaccurate in measurement result and has no compatibility with a longer line are overcome, the measurement accuracy is improved, the fault can be accurately positioned within the range of 40m, the method has remarkable superiority, and the reliability and the remarkable superiority of the distance measurement method are verified by building a single-ended direct-current power distribution network simulation model based on a modular multilevel current converter (MMC) on MATLAB/PSCAD.

Drawings

Fig. 1 is a schematic diagram of a feeder ranging module.

FIG. 2 is a left side current diagram of a single pole ground fault

FIG. 3 is an equivalent circuit diagram at the time of a single-pole ground fault

FIG. 4 is a schematic view of a current discharge waveform

FIG. 5 is a waveform of 1-range module discharge current

FIG. 6 is a discharge current waveform diagram for the 2-range module

FIG. 7 is a fitting result of the discharging current of the 1-terminal ranging module without considering the distributed capacitance

FIG. 8 is a fitting result of the discharge current of the 2-terminal ranging module without considering the distributed capacitance

FIG. 9 is a waveform of a discharging current of a 1-terminal ranging module in consideration of distributed capacitance

FIG. 10 is a waveform of the discharging current of the 2-terminal ranging module in consideration of the distributed capacitance

FIG. 11 is a fitting result of the discharging current of the 1-terminal ranging module in consideration of the distributed capacitance

Fig. 12 is a fitting result of the 2-terminal ranging module discharge current considering the distributed capacitance.

Detailed Description

In order to facilitate the understanding of the technical solutions of the present invention for those skilled in the art, the technical solutions of the present invention are further described below with reference to the accompanying drawings and the embodiments.

In this embodiment, the feeder line ranging module is as shown in fig. 1. The method comprises the following specific steps:

(1) the distance measurement module is arranged at the head ends of the positive feeder line and the negative feeder line of the fault feeder line, and the distance measurement modules at the two ends are switched in or out according to the following requirements.

Is a distance measuring mode capacitor;

a distance measurement module inductor;

，

，

，

is a single-pole three-throw switch. When the circuit breaker is in normal operation, the circuit breaker switch is closed, and the single-pole three-throw switches of the ranging modules are respectively connected to

Terminal and

and the direct current power supply and the capacitor are connected to keep the voltage of the capacitor.

Assuming a single-pole earth fault on the positive pole, the circuit breaker is opened first, with a delay of 0.5s

The switch is connected to

Terminal, switch

Is connected to

And the end puts the ranging module into a fault line. The delay of 0.5s is to avoid the influence of transient current generated when the breaker is opened on the fault location process. 1 s rear switch

，

And restoring, and connecting with the direct-current power supply again to charge the capacitor so as to prepare for next fault location. And (3) the 2-end distance measurement module is put into measurement after 1.5s, and the operation is the same as that when the 1-end distance measurement module is put into measurement. The time difference of the two end ranging modules is 0.5s, so that the current of one end ranging module is prevented from influencing the fault ranging of the other end。

(2) Formula for calculating discharge current by using RLC charge-discharge circuit

Assuming that a single-machine ground fault occurs in the positive line in fig. 1, since the capacitance to ground of the dc line is small and the influence on the ranging accuracy is small, the dc line can be equivalent to a resistance-inductance model, and the current path and the equivalent circuit of the single-pole ground fault are respectively shown in fig. 2 and fig. 3.

In FIG. 1

Is the inductance per unit length of the direct current line,

is the resistance per unit length of the direct current line,

in order to be the transition resistance, the resistance,

the distance from the fault point to the ranging module.

Referring to fig. 3, when the ranging module operates, the switch of fig. 3

Closed, switch off

When the circuit is opened, current flows in the circuit, and a circuit equation is written by respectively considering the voltage-current relationship of the inductor and the capacitor to obtain a second-order differential equation related to the discharge current, namely

In the formula:

is time;

is the loop current;

and

the total resistance and the total inductance of the loop, respectively.

Inductance and capacitance parameters in the distance measurement module are adjusted to enable the circuit in the figure 3 to meet the under-damped oscillation condition, and the discharge current expression obtained by solving the formula (1) is as follows:

in the formula:

is a coefficient;

and

(3) And extracting the line current and carrying out nonlinear least square fitting on the current data to obtain relevant parameters in the discharge current expression, and calculating the fault distance according to the relevant parameters.

The discharge current waveform is shown in FIG. 4, and the damped oscillation frequency of the current loop can be obtained by performing nonlinear least squares fitting on the current data

Damping constant of

And

。

resonant angular frequency

And

a relationship of (A) and

distance to fault

Can be expressed as:

the fault distance can be obtained by the connected solution

Comprises the following steps:

in the formula:

the total length of a direct current distribution network line;

and

The parameters obtained by simultaneous double-end measurement can eliminate the inductance and resistance parameters of the line, and obtain a fault distance expression expressed by the total length L of the line, namely

The process will be further illustrated with reference to specific examples below:

a single-ended direct-current power distribution network established based on a modular multilevel current-to-current converter (MMC) is provided with 10km positive and negative bus voltages of +/-10 kv respectively, and an alternating-current measuring transformer adopts

The connection, star side grounding mode, direct current side grounding mode through clamping resistance.

Because the invention mainly studies the measurement of the fault distance, all analysis studies are carried out on the basis of the known fault line; when the feeder line has a short circuit due to a ground fault, all fault feeder lines are considered to be automatically cut off;

because distance measuring module is all installed to positive negative pole feeder side head end, another distance measuring module of a distance measuring module during operation promptly works in the mode of charging, makes when distance measuring module puts into theoretically, and the electric current of trouble feeder must approximately satisfy theoretical analysis's current discharge formula, promptly:

because the influence of distributed capacitance exists when a circuit has a unipolar ground fault, the obtained actual current waveform is not the same as the theoretical current waveform obtained by a current discharge formula of theoretical analysis, and therefore least square fitting needs to be carried out on the actual current waveform to calculate the damped oscillation frequency of the current loop

、

Damping constant

、

And

、

and substituting the parameters into a single-end distance measurement formula and a double-end distance measurement formula respectively:

in the actual measurement process, the influence of line inductance and resistance exists, so that the result of single-end measurement obviously has errors, for double-end distance measurement, the principle of mathematical symmetry is utilized, the influence of the line inductance and the resistance is mutually counteracted, and the errors caused by the influence are overcome, so that the double-end distance measurement has more superiority and reliability compared with the single-end distance measurement.

The following analysis is performed in conjunction with an actual simulation waveform example:

the line and ranging module parameters are shown in the following table:

table 1 simulation parameter settings

Inductance and electric capacity in the distance measuring module are set to meet the second-order circuit under-damping condition, namely:

the smaller the left part of the inequality is, the larger the right part is, and the easier the under-damping condition is to be satisfied. For the left side of the inequality, when a fault occurs at the end of a line, the resistance of the faulty line is the resistance of the line at the total length, and the left side of the inequality is the resistance of the line at the time

The maximum value, as can be seen from Table 1, is 1.2

. High-resistance ground faults of the power distribution network mostly occur in mountainous and hilly areas, special research for high-resistance ground fault distance measurement is provided, while ground faults in the urban power distribution network are metallic and low-transition-resistance ground faults generally, and the fault resistance value is small. E.g. transition resistance less than 15

Then the left side of the inequality is less than 20

. For the right side of the inequality, when a fault occurs at the beginning of the line,

can be ignored, the right value of the inequality is minimum at the moment, the inductance value of the distance measuring module is set to be 100 times of the capacitance value, and therefore the right size is 20

At this time, the inequality holds. When the line is not generatedAt the beginning of the line, because

The right part is larger, the inequality holds. In order to improve the distance measurement precision, the inductance of the distance measurement module and the unit inductance of the direct current line are set to be the same order of magnitude, and one hundredth of the inductance is taken as the capacitance of the distance measurement module. The inductance and capacitance setting values of the ranging module are shown in table 1.

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 during the fault occurrence is considered, so 3 conditions of not considering the distributed capacitance, considering the distributed capacitance and the change of the transition resistance need to be verified, and the method is compared and analyzed with other methods to verify the correctness and the validity of the method.

1. Fault location without consideration of distributed capacitance

When the distributed capacitance of a direct current line is not considered, the actual distance d between a single-pole grounding fault point and a direct current distribution network model ranging module is assumed to be 4km, and the transition resistance is assumed to be 6

The discharge current waveform of the distance measuring module is obtained as a smooth oscillation attenuation sine wave, as shown in figure 7 and figure 8,

after the discharge current data is obtained through the distance measurement module, nonlinear least square fitting is carried out on the discharge current data to obtain an attenuation coefficient

He Wei

130.2 and 96.55, respectively, damped oscillation frequency

And

1304rad/s and 1121 rad/s. Such asAs shown in fig. 8, the measured discharge current substantially coincides with the fitted discharge current, and the fitting effect is good, and the fitting schematic diagram is as follows:

will be provided with

，

，

，

And substituting the distance between the fault point and the ranging module into a double-end ranging formula to obtain the distance of 3.9834 km. The fault ranging error percentage is 0.166%. The distributed capacitance is ignored in simulation, so that the simulation is an ideal situation, and therefore, errors are almost eliminated.

2. Fault location when considering distributed capacitance

The distributed capacitance exists in the actual distribution line, and the equivalent model in the line is

The actual distance d between the assumed monopole grounding fault point and the DC distribution network model ranging module is 4km, and the transition resistance is 6

Distributed capacitance of 0.618

The/km, at this time, the discharging current waveform diagram of the ranging module is shown in fig. 9 and 10:

as can be seen from fig. 9 and 10, 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 nonlinear least square fitting to obtain the attenuation coefficient

And

120.2 and 83.88, respectively, damped oscillation frequency

And

are 1276rad/s and 1122 rad/s. As shown in fig. 11 and 12, the measured discharge current substantially agrees with the fitted trend.

Will be provided with

，

，

，

Substituting into a double-end distance measurement formula to obtain that the distance x between the fault point and the distance measurement module is 3.9833km and the percentage of the distance measurement error

Comprises the following steps:

this error percentage meets the accuracy requirements for fault location. To further verify the effectiveness of the proposed fault location method, simulation experiments were performed herein for different fault distances, and the corresponding ranging errors are listed in table 2, where the actual fault distance and the fault ranging distance both represent the distance of the fault point from the ranging module.

TABLE 2 error percentage for fault ranging at different distances

As can be seen from Table 2, for the DC line model considering the distributed capacitance, the positioning method still has higher ranging accuracy, and for the 10km distribution line, the error percentage is less than 4%, and the requirement of fault positioning is met.

3. Fault location while accounting for 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 ranging module is 3km, considering the influence of distributed capacitance, the ranging distances and error percentages of different transition resistances at the same fault distance are listed 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%. Since the transition resistance of the urban DC distribution line is generally metallic and small, the value is not large, so when the transition resistance is large value 15

In the time, the ranging error percentage at the fault distance of 3km is 0.478%, the error percentage is the largest, and the ranging precision is higher at the moment and still meets the ranging precision requirement. Therefore, the method provided by the invention can always keep higher positioning accuracy under the condition of considering the change of the transition resistance, and meets the requirement of fault location.

From the simulation results of the three cases in the example, the following conclusions can be drawn:

(1) the method has higher distance measurement precision under the actual condition of considering the changes of the distributed capacitance and the transition resistance, and has practical application value.

(2) The method is suitable for the condition that the transition resistance in the urban power distribution network is small, the distance measurement module can repeatedly measure, accidental conditions are avoided, and the reliability and accuracy of distance measurement are greatly improved.

The embodiments are only for illustrating the technical idea of the present invention, and the technical idea of the present invention is not limited thereto, and any modifications made on the basis of the technical scheme according to the technical idea of the present invention fall within the scope of the present invention.

Claims

1. A single-ended direct current distribution line single-pole ground fault distance measurement method is characterized in that a distance measurement module is respectively arranged at the head ends of a positive feeder line and a negative feeder line to measure the distance of a fault feeder line, the least square fitting is carried out on current waveforms respectively measured by the two distance measurement modules to obtain respective corresponding parameters, and the parameters are substituted into a theoretical fault measurement distance formula to obtain a theoretical fault distance value.

2. The single-ended direct-current distribution line unipolar ground fault distance measurement method according to claim 1, characterized in that the distance measurement module is composed of an inductor, a capacitor, a direct-current power supply and a switch, the distance measurement module and a fault line form a second-order discharge circuit of underdamped oscillation by adjusting the sizes of the inductor and the capacitor, the oscillation frequency and the attenuation coefficient of discharge current attenuation are determined by nonlinear least square fitting, a fault distance measurement algorithm in a traditional injection method is improved by introducing double-end information quantity, a fault distance calculation formula with the total line length, the inductance and the capacitance of the distance measurement module as parameters is obtained, and the influence of the inductor and the resistance of the direct-current line on fault location is eliminated.

3. The single-ended direct current distribution line single-pole ground fault location method of claim 1, wherein the fault distance obtained by solving in series

The calculation formula of (2) is as follows:

in the formula:

the total length of a direct current distribution network line;

and