CN103091606A - Grounding fault detecting method for direct current system with high anti-interference capacity - Google Patents

Abstract

The invention discloses a grounding fault detecting method for a direct current system with high anti-interference capacity, and belongs to the field of electric power detection. The grounding fault detecting method aims to solve the problems that when an alternating current signal injection method is adopted to monitor a grounding fault of the direct current system and interference exists, measurement is not accurate and misjudgment is caused. The grounding fault detecting method comprises the following steps of injecting low-frequency alternating current signals between a direct current system bus and the ground at a set interval T, wherein the frequency of the signals is lower than that of the power frequency, and the T is 1 second-5 seconds; measuring and obtaining current signals in a determined fault branch circuit and obtaining signals of all frequency components of the current signals by the utilization of a harmonic wave extraction unit and by the adoption of the windowing interpolation method; comparing the low-frequency alternating current signals injected in step 1 with the signals of all the frequency components of the current signals obtained in step 2 to determine the positions where the signals of the frequency components, corresponding to the injected low-frequency alternating current signals, disappear. The positions are fault points.

Description

The Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity

Technical field

The present invention relates to the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity, belong to the electric power detection field.

Background technology

Direct supply is widely used in generating plant and transformer station.Under normal condition; it is powered to dc loads such as indicating relay, signal lamp, contactor coils; breaking coil, closing coil power supply to isolating switch electromagnetism operating mechanism; when AC power breaks down; for the closing coil of relay protection and aut.eq., isolating switch, breaking coil, carrier communication apparatus etc. provide working dc power, its effect is irreplaceable especially.Direct supply plays an important role to the safe and reliable operation of electric system.

One point earth is one of modal insulation breakdown fault of straight-flow system, if can not in time find and get rid of earth point, when multipoint earthing occurring in system, easily causes the serious consequences such as DC load cisco unity malfunction, relay misoperation work.According to transformer station and power plant's straight-flow system design national standard, straight-flow system relay, control loop etc. can guarantee that the grounding resistance of normal operation should be not less than 20k Ω, therefore judges that the standard of dc system grounding is mainly whether grounding resistance is less than 20k Ω.

Main employing bridge method and AC signal injection method are monitored the earth fault of straight-flow system at present.The ultimate principle of bridge method is that straight-flow system both positive and negative polarity ground connection equivalent resistance and artificial two resistance that arrange are formed electric bridge, when system is normal, and bridge balance, when certain some ground connection appearred in system, the electric bridge out of trim produced alerting signal.The method circuit is simple; with low cost; but still can not Normal Alarm to some situation, mainly, control, relay protection few in a way of the method at present, the aut.eq. loop is simple, automatization level is low and use more to power supply reliability less demanding small plant.The AC signal injection method adopts following principle to realize usually: regularly inject low-frequency ac electric current signal between straight-flow system bus and the earth, measure on the fault branch of determining and obtain current signal, the place that the AC signal respective frequencies component signal of injection disappears is the trouble spot.The method does not have the shortcoming of bridge method, is the main stream approach of monitoring at present the earth fault of straight-flow system.

The key point of AC signal injection method is whether the AC signal respective frequencies component signal that judgement is injected disappears, disturb more owing to collecting signal, especially occur that non-integer harmonics causes non-integer-period sampled the time routine Fourier algorithm result of calculation have error, there is the disconnected situation of erroneous judgement, must research be improved.

Summary of the invention

To the present invention seeks to the earth fault that adopts AC signal injection method monitoring straight-flow system in order solving, to measure inaccurately when disturbing existing, cause judging disconnected problem by accident, a kind of Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity is provided.

The Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity of the present invention, the method comprises the following steps:

Step 1, T at set intervals, between straight-flow system bus and the earth injected frequency lower than the low-frequency ac electric current signal of power frequency, T=1s～5s,

Step 2, measure on the fault branch of determining and obtain current signal, utilize harmonic extraction unit, and adopt the windowed interpolation method to obtain the signal of each frequency component in current signal,

The signal of each frequency component of the current signal that the low-frequency ac electric current signal that step 3, comparison step 1 are injected and step 2 are obtained, the place that definite low-frequency ac signal respective frequencies component signal that injects disappears, this place is the trouble spot.

In step 2, the windowed interpolation method adopts and to add Hanning window interpolation Fourier algorithm and realize, the Hanning window function is:

$w\left(n\right)=\frac{1}{2}-\frac{1}{2}\cos \left(\frac{2\mathrm{\π}}{N}n\right),n=\mathrm{0,1},...,N-1---\left(1\right)$

N is natural number;

The process that adopts the windowed interpolation method to obtain the signal of each frequency component in current signal is:

Step 1, the current signal DFT result of signal after adding the Hanning window is:

$X_w\left(n\right)=\frac{1}{2}X\left(n\right)-\frac{1}{4}X(n-1)-\frac{1}{4}X(n+1)---\left(2\right)$

In formula: X (n) is the DFT result of stray currents signal x (n);

The first-harmonic of step 2, current signal or harmonic frequency f obtain by following formula:

f=(k+△k)△f（3）

In formula: △ f=1 △ T, △ T are the current signal sampling duration;

K is sampling number; K is integer; △ k is decimal, and obtains by following formula:

Harmonic amplitude B after the first-harmonic of step 3, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

B _k=B' _k2π△k(1-△k ²)sin(△kπ)（5）

In formula: B' _kFirst-harmonic or the harmonic frequency f of the current signal that obtains for step 2 carry out Fourier transform gained harmonic amplitude,

Phase angle theta after the first-harmonic of step 4, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

θ _k=angle(X _w(k)exp(-j△k(1-1N)π))（6）

J is imaginary unit, and angle (z) is used for obtaining the phase angle of plural z;

Harmonic amplitude B after step 5, the correction of obtaining according to step 3 _kPhase angle theta after the correction of obtaining with step 4 _kObtain the signal of each frequency component in current signal.

Advantage of the present invention:

1, accuracy of computation is high: based on the amplitude that adds Hanning window interpolation Fourier algorithm and can accurately obtain Injection Signal.

2, antijamming capability is strong: can effectively reduce based on adding Hanning window interpolation Fourier algorithm the error that non-integer harmonics is brought to localization of fault.Therefore, antijamming capability is strong.

Description of drawings

Fig. 1 is the process flow diagram of the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity of the present invention.

Embodiment

Embodiment one: below in conjunction with Fig. 1, present embodiment is described, the Detection Method for Earthing Fault of HVDC System of the described high anti-jamming capacity of present embodiment, the method comprises the following steps:

Step 1, T at set intervals, between straight-flow system bus and the earth injected frequency lower than the low-frequency ac electric current signal of power frequency, T=1s～5s,

Step 2, measure on the fault branch of determining and obtain current signal, utilize harmonic extraction unit, and adopt the windowed interpolation method to obtain the signal of each frequency component in current signal,

The signal of each frequency component of the current signal that the low-frequency ac electric current signal that step 3, comparison step 1 are injected and step 2 are obtained, the place that definite low-frequency ac signal respective frequencies component signal that injects disappears, this place is the trouble spot.

During synchronized sampling, each harmonic component is corresponding each frequency discrimination point just, and gained frequency, amplitude, phase place are exact value.Become when due to the power system signal frequency being, and the restriction that is subject to hardware is difficult to dynamic sampling rate adjusting and realizes synchronized sampling completely.In the non-synchronous sampling situation, signal frequency is between two spectral lines, and frequency, amplitude and the phase angle of discrete Fourier transformation (Discrete Fourier Transform, DFT) gained are all inaccurate, need to proofread and correct.Thereby can ask for the main lobe centre coordinate according to spectral line in main lobe and obtain frequency, amplitude and phase place accurately, can further improve the accuracy of above calculating by adding suitable window function.

Window function has a variety of, and present embodiment selects the Hanning window to be because the method has been taken into account frequency analysis effect preferably, calculated amount is little, speed is fast, realization is easy.

If adopt conventional discrete fourier algorithm, the non-integer harmonics that exists in electric system can affect the judgement of Injection Signal amplitude, adopts the Fourier algorithm that adds Haning window interpolation can avoid this point.The Design of DC System Grounding Fault Detection system of the high anti-jamming capacity of realizing comprises following function:

1) at set intervals between straight-flow system bus and the earth injected frequency lower than the low-frequency ac electric current signal of power frequency.

2) measure the acquisition current signal on the fault branch of determining.

3) employing windowed interpolation Fourier algorithm obtains the signal of each frequency component, obtains the amplitude of the corresponding harmonic wave of Injection Signal frequency.

4) place that the AC signal respective frequencies component signal that injects disappears is the trouble spot.

Embodiment two: present embodiment is described further embodiment one, and the described low-frequency ac electric current of step 1 signal adopts the Low Frequency Sine Signals generator to send.

Embodiment three: present embodiment is described further embodiment one or two, and in step 2, the windowed interpolation method adopts and to add Hanning window interpolation Fourier algorithm and realize, the Hanning window function is:

$w\left(n\right)=\frac{1}{2}-\frac{1}{2}\cos \left(\frac{2\mathrm{\π}}{N}n\right),n=\mathrm{0,1},...,N-1---\left(1\right)$

The process that adopts the windowed interpolation method to obtain the signal of each frequency component in current signal is:

Step 1, the current signal DFT result of signal after adding the Hanning window is:

$X_w\left(n\right)=\frac{1}{2}X\left(n\right)-\frac{1}{4}X(n-1)-\frac{1}{4}X(n+1)---\left(2\right)$

In formula: X (n) is the DFT result of stray currents signal x (n);

The first-harmonic of step 2, current signal or harmonic frequency f obtain by following formula:

f=(k+△k)△f（3）

In formula: △ f=1 △ T, △ T are the current signal sampling duration;

K is sampling number; K is integer; △ k is decimal, and obtains by following formula:

Harmonic amplitude B after the first-harmonic of step 3, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

B _k=B' _k2π△k(1-△k ²)sin(△kπ)（5）

In formula: B' _kFirst-harmonic or the harmonic frequency f of the current signal that obtains for step 2 carry out Fourier transform gained harmonic amplitude,

Phase angle theta after the first-harmonic of step 4, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

θ _k=angle(X _w(k)exp(-j△k(1-1N)π))（6）

J is imaginary unit, and angle (z) is used for obtaining the phase angle of plural z;

Harmonic amplitude B after step 5, the correction of obtaining according to step 3 _kPhase angle theta after the correction of obtaining with step 4 _kObtain the signal of each frequency component in current signal.

Embodiment four: present embodiment provides a specific embodiment: the validity of this system is described as follows: the Injection Signal frequency is 20Hz, and amplitude is 1, and interfering frequency is 21.5Hz, and amplitude is 2.Obtain current signal in the 1.5km of fault branch place's collection, this place is earth fault, Injection Signal disappears at this place, when adopting conventional Fourier algorithm due to the impact of interference frequency component, the Injection Signal component of corresponding 20Hz is still 0.87, judgement is not earth point herein, and will exist error to the localization of fault of straight-flow system this moment.Use the 20Hz that obtains after algorithm of the present invention and the component of 21.5Hz to be respectively 0 and 2, consistent with truth, judgement is earth point, entirely accurate as a result herein.

Claims (4)

1. the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity, is characterized in that, the method comprises the following steps:

Step 1, T at set intervals, between straight-flow system bus and the earth injected frequency lower than the low-frequency ac electric current signal of power frequency, T=1s～5s,

Step 2, measure on the fault branch of determining and obtain current signal, utilize harmonic extraction unit, and adopt the windowed interpolation method to obtain the signal of each frequency component in current signal,

The signal of each frequency component of the current signal that the low-frequency ac electric current signal that step 3, comparison step 1 are injected and step 2 are obtained, the place that definite low-frequency ac signal respective frequencies component signal that injects disappears, this place is the trouble spot.

2. the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity according to claim 1, is characterized in that, the described low-frequency ac electric current of step 1 signal adopts the Low Frequency Sine Signals generator to send.

3. the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity according to claim 1, is characterized in that, the frequency of low-frequency ac electric current signal is 10Hz～30Hz.

4. the Detection Method for Earthing Fault of HVDC System of high anti-jamming capacity according to claim 1, is characterized in that, in step 2, the windowed interpolation method adopts and adds Hanning window interpolation Fourier algorithm and realize, the Hanning window function is:

$w\left(n\right)=\frac{1}{2}-\frac{1}{2}\cos \left(\frac{2\mathrm{\π}}{N}n\right),n=\mathrm{0,1},...,N-1---\left(1\right)$

The process that adopts the windowed interpolation method to obtain the signal of each frequency component in current signal is:

Step 1, the current signal DFT result of signal after adding the Hanning window is:

$X_w\left(n\right)=\frac{1}{2}X\left(n\right)-\frac{1}{4}X(n-1)-\frac{1}{4}X(n+1)---\left(2\right)$

In formula: X (n) is the DFT result of stray currents signal x (n);

The first-harmonic of step 2, current signal or harmonic frequency f obtain by following formula:

f=(k+△k)△f（3）

In formula: △ f=1 △ T, △ T are the current signal sampling duration;

K is sampling number; K is integer; △ k is decimal, and obtains by following formula:

Harmonic amplitude B after the first-harmonic of step 3, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

B _k=B' _k2π△k(1-△k ²)sin(△kπ)（5）

In formula: B' _kFirst-harmonic or the harmonic frequency f of the current signal that obtains for step 2 carry out Fourier transform gained harmonic amplitude,

Phase angle theta after the first-harmonic of step 4, the current signal that obtains according to step 2 or harmonic frequency f obtain and proofread and correct _k:

θ _k=angle(X _w(k)exp(-j△k(1-1N)π))（6）

J is imaginary unit, and angle (z) is used for obtaining the phase angle of plural z;

Harmonic amplitude B after step 5, the correction of obtaining according to step 3 _kPhase angle theta after the correction of obtaining with step 4 _kObtain the signal of each frequency component in current signal.

Images