CN1715937A - Detecting device and its method for DC system earthing fault - Google Patents

Abstract

The present invention discloses detecting device and method for the earthing fault in DC system. The device includes microprocessor and several current transformers. The microprocessor is connected to low frequency signal source, detecting bridge, display alarm and S/H circuit. The input of the sync link is connected to the positive and negative buses of the DC system, and the output is connected via isolating link to the input of the multiway switch. The method includes monitoring the insulating status of the DC system with the double asymmetrical bridge; judging the earthing polarity, and sending signal to the microprocessor for starting the low frequency signal source and applying stable low frequency sinusoidal or cosine to the DC system in case of finding out earthing fault; and performing looped network branch treatment when the current in the current transformers is greater than 1 A, or else performing common branch treatment; and other steps.

Description

DC System Grounding Fault Detecting Device and detection method thereof

Technical field

The present invention relates to dc system fault pick-up unit and detection method thereof in electric system Earth Fault Detection technical field, the especially electric system.

Background technology

The straight-flow system of generating plant, transformer station is a very important working power in the electric system, and it provides power supply for secondary devices such as electric, thermal technology, aut.eq., relay protection, emergency lighting, communications.The operate as normal of these secondary devices is very important for the safe operation that ensures generating plant, transformer station.Therefore, requiring its working power---straight-flow system and network thereof have the reliability of height.When straight-flow system generation one point earth fault (directly ground connection or insulation against ground resistance reduce); owing to there is not short-circuit current to flow through; fuse can not fuse; generally can not produce the harmfulness consequence immediately; but if 2 ground connection are simultaneously taken place, then may cause the misoperation of telltale, control loop and protective relaying device, cause circuit breaker trip; or directly cause the direct current operative power source short circuit, thereby cause serious power system accident.

At present, the detection that connects fault for straight-flow system mainly contains several method: traditional balanced bridge method, low frequency signal injection method, two low frequency signal injection method, dc detection method etc.Above the whole bag of tricks all has the characteristics of himself, all to a certain degree for eliminating earth fault, improves safety, the reliability of straight-flow system and has made contribution separately.Yun Hang straight-flow system is a wiring complexity in practice, the supply network of huge multiple-limb.For important load, in order to improve the reliability of its power supply, generally adopt the segments in a circular shape method for power supplying, and, be divided into separately independently power supply network according to the kind and the path of loading.The looped network mode is powered and is made the direct current network complex structure, and the difficulty of carrying out the ground connection detection becomes big.On the other hand, because the existence of a large amount of anti-interference filtration electric capacity of introducing in the distributed capacitance of circuit itself and the Automation of Electric Systems device makes the traditional detection device effectively to detect earth point.Therefore, how accurately and reliably detection of ground faults is a subject matter of current straight-flow system existence in service.

Summary of the invention

Technical matters to be solved by this invention is, a kind of pick-up unit of DC system earth fault is provided, and this method can effectively be finished in system and have Earth Fault Detection under looped network and the big ground capacitance situation.Another technical matters to be solved by this invention is, a kind of detection method of DC system earth fault is provided, and this device can effectively be finished in system and have Earth Fault Detection under looped network and the big ground capacitance situation.

For solving the problems of the technologies described above be, DC System Grounding Fault Detecting Device of the present invention comprises microprocessor and a plurality of current transformer, a plurality of current transformers are enclosed within respectively on the straight-flow system different branch, it is characterized in that described DC System Grounding Fault Detecting Device also comprises low frequency signal source, detects electric bridge, display alarm and sampling hold circuit; The output terminal of described a plurality of current transformers is connected with the input end of multi-way switch respectively, and the output terminal of multi-way switch is connected with the input end of described sampling hold circuit; Described DC System Grounding Fault Detecting Device is provided with synchronism link, and the input end of synchronism link is connected with the positive and negative busbar of straight-flow system, and the output terminal of synchronism link is connected with the input end of described multi-way switch by isolation link; The output terminal of described low frequency signal source is connected with the positive and negative busbar of straight-flow system, is used for injecting the low frequency signal voltage source to straight-flow system when earth fault; Described detection electric bridge is connected with the positive and negative busbar of straight-flow system, is used to monitor the insulation status of straight-flow system; When monitoring earth fault, judge polarity of ground, and send information to start low frequency signal source, inject stable sine or cosine letter low-frequency voltage signal to straight-flow system to microprocessor, the frequency of low frequency signal is between 10-35HZ, and the amplitude of low frequency signal is between 20-30V.

Described microprocessor is the ARM single-chip microcomputer.

Described detection electric bridge is two asymmetric electric bridges.

Described low frequency signal source is that frequency is that 30Hz, amplitude are sine or the cosine low-frequency voltage signal of 30V.

Be that Detection Method for Earthing Fault of HVDC System of the present invention may further comprise the steps for solving the problems of the technologies described above:

(1) monitor the insulation status of straight-flow system by two asymmetric electric bridges, when straight-flow system just often, bridge unit on-line real time monitoring positive and negative busbar is to geographical situation;

(2) when monitoring earth fault, judge polarity of ground, and send information to start low frequency signal source to microprocessor;

(3) inject stable sine or cosine low-frequency voltage signal to straight-flow system, the frequency of low frequency signal is between 10-35HZ, and the amplitude of low frequency signal is between 20-30V;

(4) come the detection branch current signal by the current transformer that is enclosed within each branch road, handle, handle otherwise carry out the general branch road of step (5) when the current transformer electric current carries out step (6) characteristic of loop branches during greater than 1A;

(5) general branch road is handled:

The low-frequency voltage signal that is loaded on the direct current network bus is sampled, calculate amplitude U and phase according to a kind of Fourier algorithm;

The detected branch current signal discrete of current transformer is turned to equally spaced sample sequence; And branch current signal and low-frequency voltage signal are sampled at synchronization;

Employing is carried out filter preprocessing based on the filtering method of 3 B spline wavelets conversion, filtering interfering and higher hamonic wave;

Employing extracts the amplitude and the phase place of low frequency component simultaneously based on Morlet wavelet low frequency component extraction method from the branch current signal of complexity;

According to amplitude, the phase place of the low-frequency voltage amplitude that obtains, phase place and branch road low-frequency current, calculate the branch road grounding resistance, compare with the ground connection criterion, promptly whether this branch road of decidable ground connection is taken place; In kind detect other branch roads;

(6) characteristic of loop branches is handled:

Select the Morlet small echo as the wavelet basis that the detected current signal of current transformer is carried out continuous wavelet transform, select ω for use ₀=10 Morlet small echo, and yardstick, frequency window center and the frequency supporting and space of calculating different frequency correspondence carry out continuous wavelet transform and just different frequency components are limited in certain bandwidth;

Asking the advantage of being occupied on the body surface according to box counting dimension, the solid figure of structure " sample frequency-yardstick-wavelet conversion coefficient ";

Patterned surface is asked for FRACTAL DIMENSION numerical value and camber parameter value, and fractal dimension D and camber ∧ are the functions of box length of side L;

After L was selected, corresponding ∧ (L) of each L and D (L) drew " fractal dimension-camber " point diagram on two dimensional surface;

According to the fault zone that the test of many times result determines, whether analyze this fractal-camber point in the fault zone, whether earth fault appears to judge this characteristic of loop branches.

Described low frequency signal source is that frequency is that 30Hz, amplitude are sine or the cosine low-frequency voltage signal of 30V.

Compared with prior art, the present invention has following beneficial effect: (one) the present invention can finish system effectively and have Earth Fault Detection under looped network and the big ground capacitance situation, be not subjected to the influence of complexity, huge multiple-limb supply network, improved the reliability of straight-flow system power supply; (2) the present invention adopts the insulation status of two asymmetric electric bridge monitoring straight-flow systems, has solved the identification problem that the positive and negative electrode insulation evenly descends.

Description of drawings

Fig. 1 is the functional-block diagram of DC System Grounding Fault Detecting Device of the present invention;

Fig. 2 is the testing process block diagram of Detection Method for Earthing Fault of HVDC System of the present invention;

Fig. 3-the 1st, non-characteristic of loop branches original signal;

Fig. 3-the 2nd, original signal is through 3 filtered curves of B spline wavelets;

Fig. 3-the 3rd, the curve after the Morlet small echo extracts;

Fig. 4-the 1st, the non-fault branch original signal of characteristic of loop branches fault branch and looped network;

Fig. 4-the 2nd, the 3D figure that continuous wavelet transform produces;

Fig. 4-3 fractal dimensions-camber point diagram.

Embodiment

Below in conjunction with embodiment and accompanying drawing the present invention is elaborated.

The embodiment of DC System Grounding Fault Detecting Device of the present invention:

As shown in Figure 1, DC System Grounding Fault Detecting Device of the present invention comprises microprocessor and a plurality of current transformer, and a plurality of current transformers are enclosed within respectively on the straight-flow system different branch.A plurality of current transformers among Fig. 1 are used CT1, CT2 respectively ... CTn represents.Microprocessor is senior reduced instruction microprocessor, is called for short the ARM single-chip microcomputer.Microprocessor is connected with low frequency signal source, detects electric bridge, display alarm and sampling hold circuit.The output terminal of a plurality of current transformers is connected with the input end of multi-way switch respectively, and the output terminal of multi-way switch is connected with the input end of described sampling hold circuit.DC System Grounding Fault Detecting Device is provided with synchronism link, and the input end of synchronism link is connected with the positive and negative busbar of straight-flow system, and the output terminal of synchronism link is connected with the input end of described multi-way switch by isolation link.The output terminal of low frequency signal source is connected with the positive and negative busbar of straight-flow system, is used for injecting the low frequency signal voltage source to straight-flow system when earth fault.Detecting electric bridge is two asymmetric electric bridges, and two asymmetric electric bridges are connected with the positive and negative busbar of straight-flow system, are used to monitor the insulation status of straight-flow system.When monitoring earth fault, judge polarity of ground, and send information to start low frequency signal source, inject stable sine or cosine low-frequency voltage signal to straight-flow system to microprocessor, the frequency of low frequency signal is between 10-35HZ, and the amplitude of low frequency signal is between 20-30V.What in an embodiment, low frequency signal source adopted is that frequency is that 30Hz, amplitude are sine or the cosine low-frequency voltage signal of 30V.

DC System Grounding Fault Detecting Device of the present invention is a core with ARM microprocessor S3C44BOX, and high-performance, low-power consumption are satisfied the requirement of handling small echo, fractal algorithm, and carried 10 A/D converters of hyperchannel.From the curtage signal that direct current network is gathered,, after isolating, send into multidiameter option switch through phase place and amplitude adjustment, send into A/D converter by sampling holder, finish the analysis of signal by microprocessor,, find the grounding alarm prompting in the resistance to earth value that LED shows each road.Signal source adopts the accurate waveform generator of ICL8038 to produce sinusoidal signal, and isolation link adopts the ISO124 isolated amplifier to install ground and electrical network ground isolates.

Generally, pick-up unit detects line voltage at the appointed time one time, calculate the resistance to earth of electrical network integral body by unbalanced bridge, judge whether electrical network has Abnormal Insulation, if detected resistance value is less than the regulation resistance, judge ground strap polarity, inject low frequency signal to positive and negative busbar simultaneously.Carry out patrolling and examining of grid branch subsequently, the insulation situation of judging each bar branch road is with the localization of faults.A branch road is required detection branch electric current and low-frequency voltage simultaneously, by periodic sampling, A/D conversion, the size of ARM microprocessor judges branch current, if greater than setting value, this branch road is regarded characteristic of loop branches as, handle with wavelet algorithm and fractal algorithm, judged whether fault by the complexity of analyzing current signal; If electric current is less than setting value, obtain the amplitude and the phase place of low-frequency current, voltage signal by wavelet algorithm and Fourier algorithm, calculate the resistance of branch road, judge the ground connection situation.

The embodiment of Detection Method for Earthing Fault of HVDC System of the present invention:

The present invention monitors the insulation status of straight-flow system by two asymmetric electric bridges.Straight-flow system just often, bridge unit on-line real time monitoring positive and negative busbar is to geographical situation, in case judge the one point earth fault take place, judge polarity of ground, and the information of sending immediately is to start low frequency signal source, stabilize to 30Hz to the straight-flow system injected frequency, constant amplitude is sine or the cosine low-frequency voltage signal of 30V.Come the detection branch current signal by the current transformer that is enclosed within each branch road top.Current transformer requires to have suitable no-load voltage ratio, and suitable grade is to guarantee measuring accuracy.Think that rule of thumb the CT electric current is characteristic of loop branches (owing to a large amount of harmonic circulating current composition in the looped network) greater than 1A, enter the characteristic of loop branches processing section, handle otherwise carry out general branch road.Fig. 2 is the testing process block diagram of Detection Method for Earthing Fault of HVDC System of the present invention.

One, general branch road is handled:

1. low-frequency voltage amplitude, initial phase calculate

The low-frequency voltage signal that is loaded on the direct current network bus is sampled, calculate amplitude U and phase according to a kind of Fourier algorithm.

2. branch current discretize

The detected branch current of current transformer is a continuous signal, and it is dispersed turns to equally spaced sample sequence.In order to calculate the resistance to earth of branch road, must sample to branch current signal and low-frequency voltage signal at synchronization.Fig. 3-the 1st, non-characteristic of loop branches original signal.

3. branch current pre-service

The branch current complicated component comprises multiple undesired signal, need at first carry out filter preprocessing.The requirement of filtering is should try one's best filtering interfering and higher hamonic wave, keeps the amplitude of low-frequency current and phase information undistorted again.Therefore, employing is based on the filtering method of 3 B spline wavelets conversion.Fig. 3-the 2nd, original signal is through 3 filtered curves of B spline wavelets.

For example m rank cardinal B spline function is defined as

$N_m\left(x\right)=(N_{m-1}*N_1)={\∫}_0^1{N}_{m-1}(x-t)\mathrm{dt},m\≥2---(1-1)$

In frequency domain, have:

$N_m\left(\mathrm{\ω}\right)=N_0\left(\mathrm{\ω}\right)\×N_{m-1}\left(\mathrm{\ω}\right)={\left[N_0\left(\mathrm{\ω}\right)\right]}^{m+1}={\left[\frac{\sin (\mathrm{\ω}/2)}{(\mathrm{\ω}/2)}{e}^{-\mathrm{j\ω}/2}\right]}^{m+1}---(1-2)$

When m=3, corresponding filter coefficient is

$\left\{\begin{array}{c}{h}_{-1}=0.125;h_0=0.375;h_1=0.375;h_2=0.125\\ g_0=-1.0;g_1=1.0\end{array}\right.---(1-3)$

In the formula, h is a low-pass filter function; G is the high-pass filtering coefficient.

Utilize the tower algorithm of Mallat signal decomposition can be the component on the different frequency bands, decomposition algorithm is as follows:

$\begin{array}{c}{c}_1\left(n\right)=\underset{k}{\mathrm{\Σ}}{h}_{k-2n}{c}_0\left(k\right)\\ d_1\left(n\right)=\underset{k}{\mathrm{\Σ}}{g}_{k-2n}{c}_0\left(k\right)\end{array}---(1-4)$

In the formula, c ₀(n) be sampled signal.Restructing algorithm is as follows:

$c_N\left(n\right)=\underset{k}{\mathrm{\Σ}}{h}_{n-2k}{c}_{N+1}\left(k\right)+\underset{k}{\mathrm{\Σ}}{g}_{n-2k}{d}_{N+1}\left(k\right)---(1-5)$

In the formula, N is for decomposing the number of plies.

When with frequency 1000Hz branch current being sampled, according to sampling thheorem, the highest frequency that discrete sample signals can reflect is 500Hz.Utilize formula (1-4) to do wavelet decomposition twice, be reconstructed by formula (1-5), just only comprise 0～125Hz frequency component in the signal that reconstructs, high frequency noise and higher harmonic components in the original branch current signal will be by filterings.

4. low-frequency current component amplitude, phase calculation

This is the committed step of whole testing process.Whether the low-frequency current component extraction gets accurately, is directly connected to the Calculating of Ground Resistance precision.In order from the branch current signal of complexity, to extract the amplitude and the phase place of low frequency component simultaneously, and obtain higher precision, and obtain higher precision, adopt herein based on Morlet wavelet low frequency component extraction method.

The Morlet small echo is the multiple Sine Modulated high bass wave of a kind of single-frequency, also is the most frequently used complex scalar wavelet.Its wavelet function time-domain and frequency-domain form is as follows:

Time domain $\mathrm{\ψ}\left(t\right)=(e^{i{\mathrm{\ω}}_{0}t}-e^{-{{\mathrm{\ω}}_0}^2/2})e^{-t^2/2}---(1-6)$

Frequency domain $\mathrm{\ψ}\left(\mathrm{\ω}\right)=\sqrt{2\mathrm{\π}}[e^{-{(\mathrm{\ω}-{\mathrm{\ω}}_0)}^2/2}-e^{-{{\mathrm{\ω}}_0}^2/2}\·e^{-{\mathrm{\ω}}^2/2}]---(1-7)$

Because the Morlet small echo is non-orthogonal wavelet, does not have the conjugation mirror filter, therefore, can not adopt the Mallat algorithm that signal is carried out scale-of-two wavelet decomposition and reconstruct.In actual applications, can be according to the wavelet transformation of Riemann's SERIES CALCULATION discrete series s (nT):

$\begin{array}{c}{\mathrm{WT}}_s(\mathrm{kT},a)=\frac{1}{\sqrt{a}}\·\underset{n}{\mathrm{\Σ}}s\left(\mathrm{nT}\right)\stackrel{\‾}{\mathrm{\ψ}}\left(\frac{\mathrm{nT}-\mathrm{kT}}{a}\right)\·T\\ =\frac{T}{\sqrt{a}}\·\underset{n}{\mathrm{\Σ}}s\left(\mathrm{nT}\right)e^{i{\mathrm{\ω}}_0\frac{(\mathrm{nT}-\mathrm{kT})}{a}}\·e^{-{\left(\frac{\mathrm{nT}-\mathrm{kT}}{a}\right)}^2/2}\end{array}---(1-8)$

For signal s (t)=Acos (ω _fT+ φ), the complex scalar wavelet inverse transformation formula of its discrete series is

$s\left(\mathrm{kT}\right)=\frac{2\·\mathrm{Re}\left[{\mathrm{WT}}_s(a,\mathrm{kT})\right]}{\sqrt{a}\widehat{\mathrm{\ψ}}\left(a{\mathrm{\ω}}_f\right)}=\frac{2\·\mathrm{Re}\left[{\mathrm{WT}}_s(a,\mathrm{kT})\right]}{a\·\sqrt{2\mathrm{\π}}\·e^{-{({\mathrm{\ω}}_0-{\mathrm{\ω}}_f)}^2/2}},k=\mathrm{0,1},2...N-1---(1-9)$

Get Morlet wavelet scale a=1,, centre frequency f ₀=30Hz.Morlet inverse wavelet transform algorithm by above-mentioned Morlet Wavelet Transformation Algorithm and particular form signal just can extract the low-frequency current component from branch current, calculate the amplitude and the phase place of low-frequency current then by Fourier algorithm.Fig. 3-the 3rd, the curve after the Morlet small echo extracts.

5. stake resistance calculates

According to the low-frequency voltage amplitude, the phase place that obtain in 1, with 4 in amplitude, the phase place of the branch road low-frequency current that obtains can calculate the branch road grounding resistance, compare with the ground connection criterion, promptly whether this branch road of decidable ground connection is taken place, and detects other branch roads with quadrat method.Straight-flow system voltage is generally 110 or 220V, presses electric power rules regulation, and the ground connection criterion is 25K.

Two, characteristic of loop branches is handled:

Fractal is research object with non-regular geometric form, sees through confusing unordered confusion phenomena and irregular form, discloses the essential connection that is hidden between behind rule of complicated phenomenon, part, the integral body.Because the branch road that adopts the looped network mode to move in the direct current network is not independently, it may constantly change along with the change of the method for operation, will cause the harmonic circulating current that existence is produced by looped network in the branch road, the signal content complexity.In conjunction with the fractal naturally description of fractal theory, the present invention uses the fractal dimension theory, with the method that signaling zone separates, reaches the purpose of identification signal from the angle of signal complexity.

1. characteristic of loop branches current signal pre-service

Selecting the Morlet small echo as the wavelet basis that the detected current signal of current transformer is carried out continuous wavelet transform, is 30Hz according to low frequency signal, and the current transformer sample rate current may be mixed with signal 50Hz, 100Hz, 300Hz, 600Hz and Gaussian noise.Select concrete wavelet scale: select ω for use ₀=10 Morlet small echo, and yardstick, frequency window center and the frequency supporting and space of calculating different frequency correspondence.Just can be limited in different frequency components in certain bandwidth with continuous wavelet transform, help next step fractal carrying out.Fig. 4-the 1st, the non-fault branch original signal of characteristic of loop branches fault branch and looped network.

2. the patterned surface box counting dimension calculates

Asking the advantage of being occupied on the body surface according to box counting dimension, the solid figure of structure " sample frequency-yardstick-wavelet conversion coefficient " can not isolated the relation between the signal like this, but be considered this integral body of signal all the time.

To the solid figure surface that obtains, adopt the box counting dimension method to calculate fractal dimension.Selecting the length of side is the box of L, and N (L) is the quantity of box.Estimate that N (L) is a committed step.(m, L) expression: m is put in the length of side is probability in the box of L to definition P.This box is the center with a point of fractal surface.To P (m, L) carry out following standardization:

$\underset{m=1}{\overset{N}{\mathrm{\Σ}}}P(m,L)=1---(1-10)$

N is possible counting in the box.Make the point (pixel of image) of S presentation image,, have m some box quantity therein to be so if image is covered by the box of the length of side by L:

$\left(\frac{S}{m}\right)P(m,L)---(1-11)$

Therefore, total box quantity of the whole image of expectation covering is:

$<N\left(L\right)>=\underset{m=1}{\overset{N}{\mathrm{\&Sigma;}}}\frac{S}{m}P(m,L)=S\underset{m=1}{\overset{N}{\mathrm{\&Sigma;}}}\frac{1}{m}P(m,L)$

Get box length of side L=3 respectively, 5,7,9,11, and yardstick and small echo variation factor amplified, obtain the box counting dimension under the different length of sides, obtain the figure fractal dimension by least square curve fitting.Fig. 4-the 2nd, the 3D figure that continuous wavelet transform produces.

3. patterned surface is asked for the camber parameter value;

Introduce camber parameter ∧, ∧ determines the camber on image surface, its feature description the speed of fractal mass change.Similar with the method for measuring shore line length, fractal sets depends on the length of measurement standard, and follows following energy rule:

M(L)＝kL ^D (1-12)

$M\left(L\right)=\underset{m=1}{\overset{N}{\mathrm{\&Sigma;}}}\mathrm{m\; P}(m,L)---(1-13)$

$M^2\left(L\right)=\underset{m=1}{\overset{N}{\mathrm{\&Sigma;}}}{m}^{2}P(m,L)---(1-14)$

$\mathrm{\&Lambda;}\left(L\right)=\frac{M^2\left(L\right)-{\left[M\left(L\right)\right]}^2}{{\left[M\left(L\right)\right]}^2}---(1-16)$

M (L) is the function of L, calculate ∧ (L) based on P (m, L); Fractal dimension D and camber ∧ are the functions of box length of side L.

4. draw dimension-camber figure

After L was selected, corresponding ∧ (L) of each L and D (L) drew " fractal dimension-camber " point diagram on two dimensional surface.According to the fault zone that the test of many times result determines, whether analyze this fractal-camber point in the fault zone, whether earth fault appears to judge this characteristic of loop branches.Fig. 4-3 fractal dimensions-camber point diagram.

The invention has the advantages that the accurate detection that can solve branch road earth fault under the situation that direct current network exists looped network and big ground capacitance.

Claims (6)

1, a kind of DC System Grounding Fault Detecting Device, comprise microprocessor and a plurality of current transformer, a plurality of current transformers are enclosed within respectively on the straight-flow system different branch, it is characterized in that described DC System Grounding Fault Detecting Device also comprises low frequency signal source, detects electric bridge, display alarm and sampling hold circuit; The output terminal of described a plurality of current transformers is connected with the input end of multi-way switch respectively, and the output terminal of multi-way switch is connected with the input end of described sampling hold circuit; Described DC System Grounding Fault Detecting Device is provided with synchronism link, and the input end of synchronism link is connected with the positive and negative busbar of straight-flow system, and the output terminal of synchronism link is connected with the input end of described multi-way switch by isolation link; The output terminal of described low frequency signal source is connected with the positive and negative busbar of straight-flow system, is used for injecting the low frequency signal voltage source to straight-flow system when earth fault; Described detection electric bridge is connected with the positive and negative busbar of straight-flow system, is used to monitor the insulation status of straight-flow system; When monitoring earth fault, judge polarity of ground, and send information to start low frequency signal source, inject stable sine or cosine low-frequency voltage signal to straight-flow system to microprocessor, the frequency of low frequency signal is between 10-35HZ, and the amplitude of low frequency signal is between 20-30V.

2, a kind of DC System Grounding Fault Detecting Device according to claim 1 is characterized in that, described microprocessor is the ARM single-chip microcomputer.

3, a kind of DC System Grounding Fault Detecting Device according to claim 1 is characterized in that, described detection electric bridge is two asymmetric electric bridges.

4, a kind of DC System Grounding Fault Detecting Device according to claim 1 is characterized in that, described low frequency signal source is that frequency is that 30Hz, amplitude are sine or the cosine low-frequency voltage signal of 30V.

5, a kind of Detection Method for Earthing Fault of HVDC System is characterized in that, may further comprise the steps:

(1) monitor the insulation status of straight-flow system by two asymmetric electric bridges, when straight-flow system just often, bridge unit on-line real time monitoring positive and negative busbar is to geographical situation;

(2) when monitoring earth fault, judge polarity of ground, and send information to start low frequency signal source to microprocessor;

(3) inject stable sine or cosine low-frequency voltage signal to straight-flow system, the frequency of low frequency signal is between 10-35HZ, and the amplitude of low frequency signal is between 20-30V;

(4) come the detection branch current signal by the current transformer that is enclosed within each branch road, handle, handle otherwise carry out the general branch road of step (5) when the current transformer electric current carries out step (6) characteristic of loop branches during greater than 1A;

(5) general branch road is handled:

The low-frequency voltage signal that is loaded on the direct current network bus is sampled, calculate amplitude U and phase according to a kind of Fourier algorithm;

The detected branch current signal discrete of current transformer is turned to equally spaced sample sequence; And branch current signal and low-frequency voltage signal are sampled at synchronization;

Employing is carried out filter preprocessing based on the filtering method of 3 B spline wavelets conversion, filtering interfering and higher hamonic wave;

Employing extracts the amplitude and the phase place of low frequency component simultaneously based on Morlet wavelet low frequency component extraction method from the branch current signal of complexity;

According to amplitude, the phase place of the low-frequency voltage amplitude that obtains, phase place and branch road low-frequency current, calculate the branch road grounding resistance, compare with the ground connection criterion, promptly whether this branch road of decidable ground connection is taken place; In kind detect other branch roads;

(6) characteristic of loop branches is handled:

Select the Morlet small echo as the wavelet basis that the detected current signal of current transformer is carried out continuous wavelet transform, select ω for use ₀=10 Morlet small echo, and yardstick, frequency window center and the frequency supporting and space of calculating different frequency correspondence carry out continuous wavelet transform and just different frequency components are limited in certain bandwidth;

Asking the advantage of being occupied on the body surface according to box counting dimension, the solid figure of structure " sample frequency-yardstick-wavelet conversion coefficient ";

Patterned surface is asked for FRACTAL DIMENSION numerical value and camber parameter value, and fractal dimension D and camber ∧ are the functions of box length of side L;

After L was selected, corresponding ∧ (L) of each L and D (L) drew " fractal dimension-camber " point diagram on two dimensional surface;

According to the fault zone that the test of many times result determines, whether analyze this fractal-camber point in the fault zone, whether earth fault appears to judge this characteristic of loop branches.

6, a kind of Detection Method for Earthing Fault of HVDC System according to claim 5 is characterized in that, described low frequency signal source is that frequency is that 30Hz, amplitude are sine or the cosine low-frequency voltage signal of 30V.

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