CN113176474B - Double-end direct-current power distribution network fault positioning method based on current self-adaptive control - Google Patents
Double-end direct-current power distribution network fault positioning method based on current self-adaptive control Download PDFInfo
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Abstract
The invention discloses a fault positioning method of a double-end direct-current power distribution network based on current self-adaptive control, which is characterized by comprising the following steps of: step S1: constructing a double-end flexible direct-current power distribution network model; step S2: fault detection and countermeasures; step S3: sampling a system; step S4: obtaining a calculation formula; step S5: the final fault distance is calculated. The fault positioning method utilizes a double-end method to eliminate the transition resistance, has higher capacity of resisting transition resistance interference, adopts self-adaptive control of fault current, extracts the actual fault current at the moment of switching the control mode as the peak value of a reference signal, keeps the zero crossing point of the reference signal unchanged, can greatly shorten the adjustment time of P I, improves the accuracy of active control of the current, and can effectively reduce errors among different sampling points and greatly improve the fault positioning precision by the linear regression analysis of the numerator-denominator unitary elements of the traditional double-end method fault positioning.
Description
Technical Field
The invention relates to a flexible direct-current power distribution network fault detection technology, in particular to a double-end direct-current power distribution network fault positioning method based on current self-adaptive control.
Background
In recent years, with the development of power electronic technology, a flexible direct-current power distribution network becomes an important development direction of urban intelligent power distribution in the future by virtue of the advantages that the flexible direct-current power distribution network has low line manufacturing cost and low loss, can realize independent control of active power and reactive power, has large electric energy transmission capacity, can reduce the intermediate links of connecting a photovoltaic distributed power supply, a wind power distributed power supply and a direct-current load into a power grid, further reduces the connection cost, improves the power conversion efficiency, improves the electric energy quality and the like.
At present, the established flexible direct current power distribution project mostly adopts a cable line with lower operation and maintenance cost and higher reliability to transmit electric energy. Although the reliability of power supply of the power distribution network can be improved, the cable line fails, often permanently. And cable lines in cities are mostly paved underground, which brings certain difficulty for troubleshooting. Therefore, a fault position needs to be determined by a fault accurate positioning method, and the maintenance difficulty is reduced.
According to different fault information, the fault positioning method of the flexible direct-current power distribution network can be divided into a single-end method and a double-end method. The single-end method utilizes the fault information of the single-end converter station to perform fault location, and can be specifically divided into a traveling wave method, an active injection method and a fault analysis method. The traveling wave method measures the time difference of one round trip between the detection end and the fault point by identifying the traveling wave head to calculate the fault distance, and theoretically has high measurement precision without being influenced by the line type, the transition impedance and the fault type. The traveling wave method is suitable for high-voltage direct-current transmission lines, and has larger positioning error for power distribution networks with shorter direct-current lines. The active injection method utilizes an additional module or a converter station to inject voltage or current signals to a fault point, an RLC model is built, fault information in a fault current attenuation coefficient is extracted to be located, but the initial transient state quantity of the fault of the flexible direct-current power distribution network is complex, and the attenuation coefficient is difficult to extract. The fault analysis method performs fault location by plotting a relationship between a fault distance and an electrical quantity. However, the effects of transition resistance, uneven distribution of line parameters, and differential term calculation errors cannot be ignored. The double-end method can eliminate the influence of the transition resistance on the distance measurement precision, the fault distance is calculated once by using the information of each sampling point, but the denominator term introduced by the double-end method has a zero crossing point, and the problem of numerical value stability is caused. Aiming at the situation, a double-end direct-current power distribution network fault positioning method based on current self-adaptive control is provided.
Disclosure of Invention
The invention aims to provide a double-end direct-current power distribution network fault positioning method based on current self-adaptive control, which can effectively solve the problems, combines the high controllability of a half-and-full-hybrid Modular Multilevel Converter (MMC), switches a double-end converter station into a fault current control mode simultaneously after detecting a direct-current fault, a fault current reference signal is a direct-current signal attenuated by a specific slope, and the actual values of the fault currents at the time of detecting the fault occurrence are different under different transition resistors and fault distances, so that the adoption of the specific reference signal can prolong the adjustment time of a PI controller, the self-adaptive control of the fault current is considered for reducing the PI adjustment time, the actual fault current at the time of switching the control mode is taken as the peak value of the reference signal, the zero crossing point of the reference signal is still unchanged, and then the transition resistor is eliminated by using a double-end method, and (3) obtaining the numerator denominator of a fault distance calculation formula by a double-end method, and simultaneously performing fitting processing by using a linear regression algorithm to reduce errors among different sampling points.
The purpose of the invention can be realized by the following technical scheme:
the fault positioning method of the double-end direct-current power distribution network based on the current self-adaptive control comprises the following steps:
step S1: constructing a double-end flexible direct-current power distribution network model;
step S2: fault detection and countermeasures;
step S3: sampling a system;
step S4: acquiring a calculation formula;
step S5: the final fault distance is calculated.
Further, in step S1, the converter stations 1 and 2 both use half-full hybrid MMC, and the full-bridge ratio is designed to be 50%.
Further, when a bipolar short-circuit fault occurs in the dc line in step S2, the system immediately extracts the sampled values of the output currents at the two ends of the converter stations 1 and 2 at this moment to the control system after detecting the fault, designs the peak value of the current reference signal, then linearly attenuates the current reference signal to zero, and the system controller immediately switches to the fault current control mode, so that the fault current can quickly follow the reference value under the action of the PI controller.
Furthermore, in step S3, the control system has a delay and needs time for PI adjustment, and after the delay of 1ms, the fault current and the fault voltage information at the two ends of the converters 1 and 2 are sampled and extracted, until the fault current is 0, and after the sampling is finished, the voltage and current signals at the two ends of the converters 1 and 2 are respectively calculated to be u1(t), i1(t), u2(t), and i2 (t).
Further, in the step S4, the following loop equation is obtained by considering the voltage-current relationship between the inductor and the resistor:
in the formula, r and L are respectively a line unit length resistor and an inductor, x is a distance from a converter station 1 end to a fault point, L is a total line length, Rf is a line transition resistor, and the two formulas are subtracted to obtain a fault distance:
and (3) substituting the voltage and current data of each sampling point into a fault distance expression, wherein current differential quantities di1(t)/dt and di2(t)/dt are respectively replaced by slopes k1 and k2 of reference signals, discrete points obtained by a numerator and a denominator are respectively extracted, the numerator is denoted as num (i), the denominator is denoted as den (i), the numerator is denoted by the following formula, i denotes the sampling point, and ti denotes the time of the ith sampling point.
Further, in step S5, the voltage and current data at each sampling point in step 3 are substituted into a fault distance expression, where the differential current quantities di1(t)/dt, di2(t)/dt are respectively replaced with the slopes k1, k2 of the reference signals, and the discrete data obtained by the numerator and the denominator are extracted, respectively, where the numerator is num (i) and the denominator is den (i). num (i), den (i) are shown as formula (3);
respectively carrying out unary linear regression analysis on discrete data obtained by the numerator and the denominator, fitting a prediction straight line by using a least square method, respectively recording the slopes of the numerator and the denominator prediction straight line as knum and kden, and then expressing the final fault distance as follows:
furthermore, the fault positioning method utilizes the high controllability of the hybrid MMC to actively control the current after the fault, controls the current to be attenuated by a specific slope so as to reduce the calculation error of the current differential quantity during fault positioning, and simultaneously utilizes double-end information quantity to perform fault positioning to eliminate the transition resistance, thereby avoiding the fault positioning error caused by the transition resistance.
Furthermore, the fault positioning method adopts the fault actual current at the moment of switching the control mode as the peak value of the reference signal, the zero crossing point of the reference signal is still unchanged, the adjustment time of PI (proportional integral) under different fault conditions is shortened, and the fault positioning precision is improved.
Further, in the step S4, the numerator and the denominator of the fault location formula are respectively subjected to unitary regression analysis to reduce errors between different sampling points, specifically, a least square algorithm is used to perform linear fitting on the discrete data of the numerator and the denominator to finally obtain a corresponding prediction straight line, and the slope corresponding to the prediction straight line is divided to obtain the fault distance.
The invention has the beneficial effects that:
1. according to the fault location method, the transition resistance is eliminated by using a double-end method, the high interference resistance of the transition resistance is achieved, the large calculation error is brought by calculating the fault current differential term by using the traditional double-end method, the fault current slope is constant under a specific fault, and the current differential term can be regarded as constants k1 and k2 when the fault distance is calculated on the premise that the control is stable;
2. the fault positioning method adopts the self-adaptive control of the fault current, extracts the actual fault current at the moment of switching the control mode as the peak value of the reference signal, and the zero crossing point of the reference signal is still unchanged, so that the adjusting time of the PI (proportional integral) can be greatly shortened, and the accuracy of the active control of the current is improved;
3. the fault positioning method of the invention carries out linear regression analysis on the numerator denominator unary of the fault positioning of the traditional double-end method, but not independently carrying out regression analysis on the fault voltage or current, and the integral analysis method can effectively reduce the errors among different sampling points and greatly improve the fault positioning precision.
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The invention will be further described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram of a simulation model of a double-ended DC distribution network according to the present invention;
FIG. 2 is a schematic diagram of the voltage and current flow in the case of a bipolar short circuit fault according to the present invention;
FIG. 3 is a graph showing simulation results of output currents and reference values of ports 1 and 2 of the converter station in case of a 3km fault and a metallic bipolar short circuit fault according to the present invention;
FIG. 4 is a graph of the 1.002ms-1.006ms denominator den (i), numerator num (i) sample points, and MATLAB fit results of the present invention;
FIG. 5 is a diagram of simulation results when the slope of the fault current reference signal is constant for a 50 Ω transition resistor according to the present invention;
FIG. 6 is a diagram of simulation results when current adaptive control is used with a 50 Ω transition resistor according to the present invention;
FIG. 7 is a graph of the 1.002ms-1.006ms denominator den (i), numerator num (i) sampling points, and MATLAB fitting results under current adaptive control in accordance with the present invention.
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 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.
Example (b):
referring to fig. 1 to 7, a simulation model of a double-ended dc power distribution network constructed by the fault location method of the double-ended dc power distribution network based on current adaptive control is shown in fig. 1.
The converter station 1 and the converter station 2 both adopt a semi-full hybrid structure, the full-bridge proportion is 50%, and in a steady state, the converter station 1 is set as a main station and works in a constant voltage mode, and the converter station 2 is set as a slave station and works in a constant active power mode.
And (3) fault positioning verification:
after the direct current line has the unipolar ground fault, the system can still operate for a period of time, and the fault location principle of the bipolar short-circuit fault analysis method is used for analyzing the fault location principle of the bipolar short-circuit fault which is the most common and most harmful direct current line.
The voltage and current circulation paths of the system after the bipolar short-circuit fault occurs on the direct-current side are shown in fig. 2, QS1,2,3 and 4 are closed during the fault locating period, a simulation model shown in fig. 1 is built in PSCAD/EMTDC, the rated voltage of a direct-current line is +/-10 kV, the bipolar short-circuit fault occurs on the direct-current side when the system is in 1s, the fault distance is designed to be 3km, and the transition resistance is 0.01 omega (the transition resistance is 0.01 omega, and the metallic fault can be simulated).
The application scene of the invention is double-end symmetrical semi-full mixed MMC, and the full-bridge proportion of the double-end converter station is 50%.
Assuming that a bipolar short-circuit fault occurs in a system, when the system detects the fault (about 1ms), pole controllers of converter stations 1 and 2 are immediately switched to a fault current active control mode, a fault current reference value is a straight line attenuated by a certain slope, a zero crossing point of a fault current reference signal is designed as a fixed point, a peak value of the reference signal is an actual fault current value of a line at the moment when the fault occurs, and the line fault current can quickly follow the reference value under the action of a PI (proportional-integral) controller.
And in consideration of the delay action of the control system and the adjustment time of the PI, after the delay of 1ms, sampling and extracting the fault current and fault voltage information at the two ends of the converter stations 1 and 2, and respectively calculating the voltage and current signals at the two ends of the converter stations 1 and 2 into u1(t), i1(t), u2(t) and i2(t) after the sampling is finished.
The fault location principle is as follows:
according to the fault current and voltage information, assuming that the line parameters are known, the following fault equation can be obtained by respectively considering the voltage-current relationship of the line inductance and the resistance:
in formula (1), r, L are line unit length resistance and inductance, and x is transverter 1 end to fault point distance, and L is the total length of the line, and Rf is line transition resistance, and the fault distance can be obtained by subtracting the two formulas:
and (3) substituting the voltage and current data of each sampling point into a fault distance expression, wherein current differential quantities di1(t)/dt and di2(t)/dt are respectively replaced by slopes k1 and k2 of reference signals, discrete points obtained by a numerator and a denominator are respectively extracted, the numerator is denoted as num (i), the denominator is denoted as den (i), the numerator is denoted by the following formula, i denotes the sampling point, and ti denotes the time of the ith sampling point.
And respectively carrying out unary linear regression analysis on the discrete points obtained by num (i) and den (i), fitting a prediction straight line by using a least square method, and respectively recording the slopes of the numerator prediction straight line and the denominator prediction straight line as knum and kden, wherein the final fault distance can be expressed as:
after the fault positioning principle is verified to be feasible, a 1ms system detects that a fault occurs, the converter stations 1 and 2 are immediately switched into a fault current active control mode, and a reference current is designed to be attenuated by a fixed slope;
after a delay of 1ms, i.e. 1.002s, assuming that the fault current can completely follow the reference current, the extraction of the fault voltage and current signals at the terminals 1,2 of the converter station is started, the sampling frequency is 10kHz, and the sampling cut-off time is 1.006 ms.
The simulation result of the output current of the converter stations 1 and 2 is shown in fig. 3, as can be seen from fig. 3, the PI regulator can enable the fault current to follow the reference signal within 1ms, extract the fault current and voltage information within 1.002s to 1.006s, calculate num (i) and den (i) by using the formula (3), perform unary linear regression analysis on the current in MATLAB, and the fitting result is shown in fig. 4.
Where the predicted straight-line coefficient of num (i), knum ═ 400.2. The prediction straight line coefficient of den (i), kden is-133.1, the final fault distance is 3.0068km according to the formula (4), and the fault error absolute value is 0.0068 km.
As can be seen from fig. 3, when the difference between the initial value of the reference signal and the actual value of the fault current is large, the PI regulator needs a certain time to stably control the fault current to follow the reference curve.
Assuming that the delay time after the fault is detected is 1ms, when the system is in different fault distances or transition resistances, the difference between the fault current when the fault is detected by the 1.001s system and the initial value of the reference signal may be larger, and further, the adjustment time of the PI exceeds 1ms, that is, the current does not completely follow the reference value when the fault information is extracted in 1.002 ms.
Fig. 5 shows the simulation result of the fault current when the transition resistance is 50 Ω and the fault distance is 3km, and the slope of the reference signal is still the above slope, as can be seen from the figure, when the transition resistance is large, the actual value of the output current at the two ends of the converter stations 1 and 2 is much smaller than the actual value of the reference signal, the adjustment time of the PI regulator is greatly prolonged, and the actual value of the fault current can follow the reference value even after about 4ms of adjustment, i.e. 1.005s, so that it is obvious that the fault location is early seriously disturbed.
And (3) current self-adaptive control verification:
in order to avoid the situation that the difference between the actual value of the fault current and the peak value of the reference signal is too large when the fault occurs, so that a PI regulation failure device cannot control the fault current to quickly follow the reference signal within 1ms, a fault current self-adaptive control strategy is proposed, namely when the fault is detected, the instantaneous value of the fault current at the moment is designed as the initial value of the reference signal, and thus the reference signal with the self-adaptive change of the slope can be formed.
As shown in fig. 6, where the transition resistance is 50 Ω, and the fault distance is 3km, it can be seen from the figure that, due to the adoption of the fault current adaptive control strategy, the slope of the reference signal can change along with the change of the actual current, and the output currents at the two ends of the converters 1 and 2 can be controlled to follow the reference value within 1 ms.
After the current self-adaptive control is adopted, the output currents of the ports 1 and 2 of the converter station can follow the current reference value within 1ms, the output currents and the currents of the ports are substituted into a formula, unitary linear regression analysis is carried out on the numerator and the denominator, and the fitting result is shown in fig. 7:
wherein knum ═ 80.25 and kden ═ 26.67, the fault distance was calculated according to the formula to be 3.0090km, and the fault location error absolute value was 0.009 km.
The method is obtained from the final calculation result, and has stronger anti-interference capability of the transition resistance after the current self-adaptive control is adopted.
In the description herein, references to the description of "one embodiment," "an example," "a specific example" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed.
Claims (4)
1. The double-end direct-current power distribution network fault positioning method based on current self-adaptive control is characterized by comprising the following steps of:
step S1: constructing a double-end flexible direct-current power distribution network model;
step S2: fault detection and countermeasures;
step S3: sampling a system;
step S4: obtaining a calculation formula;
step S5: calculating a final fault distance;
in the step S1, the converter stations 1 and 2 both adopt half-full hybrid MMC, and the full-bridge proportion is designed to be 50%;
when the direct current line has a bipolar short circuit fault in the step S2, after the system detects the fault, immediately extracting the sampling values of the output currents at the two ends of the converter stations 1 and 2 at this moment to the control system, designing the peak value of the current reference signal, then linearly attenuating the current reference signal to zero, and simultaneously switching the system controller to a fault current control mode immediately, wherein the fault current can quickly follow the reference value under the action of the PI controller;
in the step S3, the control system has delay and time required for PI adjustment, the fault current and fault voltage information at the two ends of the converters 1 and 2 are sampled and extracted after 1ms delay until the fault current is 0, and the sampling is finished, and the voltage and current signals at the two ends of the converters 1 and 2 are respectively counted as u 1 (t)，i 1 (t)，u 2 (t)，i 2 (t)；
In step S4, the following loop equation is obtained by considering the voltage-current relationship between the inductor and the resistor:
wherein R and L are respectively the resistance and inductance of the line unit length, x is the distance from the end of the converter station 1 to the fault point, L is the total length of the line, and R is f And for the line transition resistance, subtracting the two formulas to obtain the fault distance:
substituting the voltage and current data of each sampling point into a fault distance expression, wherein the current differential di 1 (t)/dt，di 2 (t)/dt is determined by the slope k of the reference signal 1 ，k 2 Instead, discrete points obtained by a numerator and a denominator are respectively extracted, the numerator is marked as num (i), the denominator is marked as den (i), the numerator and the denominator are respectively expressed by the following formulas, i represents a sampling point, t represents a sampling point, and i time representing the ith sample:
in step S5, a unitary linear regression analysis is performed on the discrete data obtained by the numerator and denominator, a prediction line is fitted by using a least square method, and the slopes of the numerator and denominator prediction lines are respectively recorded as k num ，k den Then the final fault distance can be expressed as:
2. the method for fault location of the double-ended direct-current power distribution network based on the current adaptive control as claimed in claim 1, wherein the fault location method utilizes the high controllability of a hybrid MMC to actively control the current after the fault, the current after the fault is controlled to be attenuated by a specific slope so as to reduce the calculation error of the current differential quantity during fault location, meanwhile, the fault location is carried out by utilizing the double-ended information quantity, the transition resistance is eliminated, and the fault location error caused by the transition resistance is avoided.
3. The method for positioning the fault of the double-end direct-current power distribution network based on the current adaptive control is characterized in that the fault positioning method adopts the actual fault current at the moment of switching the control mode as the peak value of the reference signal, the zero crossing point of the reference signal is still unchanged, the PI adjusting time under different fault conditions is shortened, and the fault positioning precision is improved.
4. The method for locating the fault of the double-end direct-current power distribution network based on the current adaptive control is characterized in that in the step S4, unitary regression analysis is respectively carried out on numerator and denominator of the fault locating formula, errors among different sampling points are reduced, linear fitting is carried out on discrete data of the numerator and the denominator by specifically utilizing a least square algorithm, a corresponding prediction straight line is finally obtained, and the slope corresponding to the prediction straight line is divided to obtain the fault distance.
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