CN1023621C - Detecting method and apparatus for earth-fault of direct-current system - Google Patents

Abstract

The present invention relates to a method for detecting ground faults in a DC system and a control device, which is especially applicable to the occasions of the DC system with great distributed capacitance. Each branch circuit is controlled and detected through a portable current probe by utilizing two low-frequency voltage signals with different frequencies, which are alternately added between a DC bus and the ground, amplitude and ratios of the currents with the two frequencies are indicated by a microcomputer discriminating instrument, and thus, fault lines and fault points are determined. The present invention has the advantages of accuracy, reliability and convenience.

Description

Detecting method and apparatus for earth-fault of direct-current system

The present invention relates to a kind of detection method and sniffer of DC system earth fault, be specially adapted to the occasion that straight-flow system has very big distributed capacitance.The straight-flow system of generating plant and transformer station provides power supply for its control, protection, signal and aut.eq., and the safe and continuous operation of straight-flow system has great importance to guaranteeing the generating power supply.Because straight-flow system is the isolated neutral system of non-grounded type,, 2 ground connection may cause that said apparatus misoperation tripping operation has a power failure, and causes a serious accident if taking place.Therefore when one point earth takes place, just should accurately promptly when guaranteeing the direct current normal power supply, detect earth point, eliminate earth fault, avoid the harm that may bring because of 2 ground connection.

Recently, both at home and abroad the detection of DC system earth fault has been carried out a lot of researchs and developed some utility units in succession.For example on dc bus, add a low frequency signal source, detect the low-frequency current of each branch road on the bus then with the clamp on amperemeter that has amplifying circuit, when the electric current of a certain branch road increases, think this branch road generation earth fault during greater than setting value.The interrupter that utilizes in dc bus symmetry loop over the ground in addition similarly produces dc pulse signal, utilizes probe to detect the changes of magnetic field detection failure branch road of each branch current.Utilize system controlled by computer that ac current signal is sampled, handles, calculated in addition, provide stake resistance resistance or fault distance.

Owing to make above-mentioned detection electric current not only comprise the ohmic fault current that will detect greatly at the long wide distributed capacitance that distributes of large-scale transformation device straight-flow system circuit but also comprise bigger capacitance current, cause to be difficult for accurately surveying line-to-ground fault.For this reason, some devices adopt different measures to try hard to solve respectively because there is the problem of being brought in big line distribution capacitance.For example: detection probe is provided with the building-out capacitor current return, relatively detects current phase and utilizes direct current magnetic potential compensation-magnetic assist conversion of positive and negative DC loop electric current etc., improves the accuracy and the sensitivity of detection.But or said apparatus need be in testing process adjusts repeatedly, or complicated operation influence detection speed, and to install the various system of quantity huge.

The objective of the invention is to propose the new method and the utility unit of a kind of DC system earth fault circuit and failure point detecting.Method and apparatus detection earth fault line of the present invention and trouble spot are not subjected to the influence of branch road ground capacitance size, belong to portable sniffer, do not need to install sensor additional again on every branch road, can accurately detect faulty line and trouble spot.

This method is alternately injected the low-frequency voltage signal of two different frequencies with the mode of regularly switching between dc bus and ground by special-purpose low-frequency voltage signal source, utilize special-purpose current probe (2) to survey the low-frequency current of each branch road again, by the relatively ratio and the amplitude of two low-frequency current signals of each branch road, can determine ground connection branch road and earth point.

The principle of this invention is:

As figure one, establishing the branch road ground capacitance is C, and two low frequency signal frequencies are f ₁And f ₂, and f ₁＞f ₂When branch road does not have earth fault, flow through this branch road capacitance current If ₁, If ₂For:

If ₁＝2πf ₁CU ……（1）

If ₂＝2πf ₂CU ……（2）

U is the low-frequency voltage value in the formula

The ratio of formula (2) and (1) is:

$\frac{I_{f_2}}{{\mathrm{<figure-callout\; id="1"\; label="I\; f"\; filenames="921048718\_DRIMG1.png"\; state="\{\{state\}\}">I\; </figure-callout>}}_{f_{}}}=\frac{{\mathrm{2\; \pi \; f}}_2\mathrm{CU}}{{\mathrm{2\; <figure-callout\; id="1"\; label="\pi \; f"\; filenames="921048718\_DRIMG1.png"\; state="\{\{state\}\}">\pi \; f</figure-callout>}}_1\mathrm{CU}}=\frac{f_2}{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="921048718\_DRIMG1.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}\mathrm{\dots \; (3)}$

This ratio is a constant of setting by the decision of the frequency of institute's making alive signal.

When this branch road during through the Rk resistance eutral grounding, this branch road ground current is:

$I^{＇}{}_{\mathrm{f\; 2}}=\sqrt{(\frac{U}{R_k}{)}^2{\mathrm{+\; (2\; \pi \; f}}_2\mathrm{CU\; )}{}^2}\mathrm{\dots \dots （5）}$

The ratio of these two electric currents is:

$\frac{I^{＇}{}_{f_2}}{I^{＇}{}_{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="921048718\_DRIMG1.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}}=\frac{\sqrt{(\frac{U}{R_k}{)}^2{\mathrm{+\; (2\pi \; f}}_2{\mathrm{CU)}}^2}}{\sqrt{(\frac{U}{R_K}{)}^2{\mathrm{+\; (2\pi f}}_1{\mathrm{CU)}}^2}}$

$=\frac{f_2}{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="921048718\_DRIMG1.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}·\frac{\sqrt{(\frac{1}{f_2}{)}^2{\mathrm{+\; (2\; \pi R}}_k{\mathrm{C)}}^2}}{\sqrt{(\frac{1}{f_1}{)}^2{\mathrm{+\; (2\pi \; R}}_k{\mathrm{C\; )}}^2}}\mathrm{\dots \dots (6)}$

Because of f ₁＞f ₂

(so 1/ (f2)) ²+ (2 π RkC) ²＞

( 1/(f ₁) ) ²+（2πRkC）

Obtain like this:

When being branch road ground connection, low-frequency current If ₂With If ₁Ratio become big, becomes big numerical value by this line mutual-ground capacitor, stake resistance and frequency difference size determine.Current ratio during by fault With just often Compare, can tell the ground connection branch road, solved the branch road ground capacitance when big, be difficult to detect the difficulty of ground connection branch road.

When the straight-flow system ground capacitance is very little, then can directly utilize probe current amplitude size decision faulty line.

After detecting the earth fault branch road, measure low-frequency current signal step by step along branch road, diminish suddenly when being checked through the current signal amplitude, this corresponding point is the trouble spot.A kind of earth fault detection device that proposes according to this method is to adapt to different straight-flow systems and different stake resistances, the low frequency signal voltage magnitude should be able to be adjustable in the 2-40 volt, the first low frequency signal frequency is at 10-20Hz, and the second low frequency signal frequency is adjustable in the 20-40Hz scope.Below in conjunction with accompanying drawing this sniffer embodiment is elaborated, wherein:

Figure one. the apparatus structure synoptic diagram

Figure two. special-purpose low frequency signal source block scheme

Figure three. portable current probe block scheme

Figure four. and microcomputer is differentiated the instrument block scheme

Figure five. special-purpose low frequency signal source principle sketch

Figure six. portable current probe schematic diagram

Figure seven. survey earth fault software logic block diagram

Be made up of three parts according to the new device that this method is made, be respectively: special-purpose low frequency signal source (1), special portable current probe (2) and microcomputer are differentiated instrument (3).Special-purpose low frequency signal source is by sine-wave generator (4), (5) and timing commutation circuit (6), voltage power amplifying circuit (7), transformer and capacitance coupling circuit (8) are formed, by (9) (10), the output of (11) output terminal, connect direct current positive bus, direct current negative busbar and the earth respectively.F among the figure five is a power amplifier, and A is a milliammeter, and V is a voltage table.Equipotential low frequency signal is added on the positive and negative dc bus simultaneously, can reduce the ripple that AC signal produces in straight-flow system.

Special portable current probe (2) is made up of clamp on amperemeter (12), capacitor compensating circuit (13), small signal amplification circuit (14).Send into microcomputer by the detected signal of probe by shielded cable (15) and differentiate instrument (3).Two capacitor C of compensating circuit (13) ₃, C ₄Same polarity connecting resistance R ₄Two ends, its effect of amp.in diode is amplitude limit and protection.The selection of component parameters is by probe characteristic, electric capacity, the decision of resistors match situation.This compensating circuit is the excitatory impedance of compensation clamp on amperemeter.

Microcomputer is differentiated instrument (3) and is made up of analog signal processing, computer system and power supply three parts.By the signal of special portable current probe (2) input, through bandpass filter (16), (17) filtering, input analog-to-digital converter (21) after amplify two utmost point amplifying circuits (18), (20), wherein (19) are to amplify range switch circuit.Computer system is by MCS-51 single-chip microcomputer (23), memory under program (24), data memory (25), parallel interface (26), nixie display (27), switching value input circuit (28), the interface circuit (22) that pilot lamp (29) and analog to digital converter (21) link to each other is formed.Power unit is formed the function that reaches portable and recharge by battery charger (30), battery pack (31), voltage stabilizer (32).

The whole system operation process is to drop into signal source (1) after straight-flow system generation earth fault, use current probe (2) and microcomputer to differentiate instrument (3) then and detect each branch road successively step by step, find out the ground connection branch road with the method for current ratio, find the trouble spot along fault wire with the way that current amplitude suddenlys change again.Ratio and amplitude are shown by charactron (27).

Figure seven is software logic block diagrams.Software has self check and moves two kinds of functions.Self check can be checked charactron, memory under program, data memory, A/D translation function, signal source frequency.Running status has the function that shows amplitude and ratio.Function is controlled by the switching value State Control, after its key is to enter running status, by sampled data is handled, calculated, distinguishes the sampled value of two kinds of different frequencies, through the ratio and the amplitude of software filtering and two signals of calculating demonstration.

The advantage of the present invention and device is to utilize ratio can solve the bigger straight-flow system of branch road direct-to-ground capacitance, utilizes the big I of amplitude to solve littler dc system grounding fault and the trouble-shooting point of branch road direct-to-ground capacitance. Portable type current probe and microcomputer are differentiated instrument by storage battery power supply, and volume is little, is easy to carry, and is accurate, sensitive, do not need at each branch road sensor installation, and cost is low.

Claims (7)

1, a kind of detection method of DC system earth fault, employing applies two low-frequency voltage signals between dc bus and ground, with each branch road of portable type current probe detection the method that applies two signal reactions is come the detection failure branch road, it is characterized in that: at first between dc bus and ground, alternately apply frequency f ₁Greater than frequency f ₂Two low-frequency voltage signals, utilize portable probe to detect the electric current I that each branch road produces two low-frequency voltage signals that applied on the bus again _F1And I _F2, differentiate instrument with microcomputer and compare the ratio of each detection branch, promptly than low-frequency current I to two different frequency signals kinetic currents _F2With the higher-frequency electric current I _F1Ratio, in the ratio that detection obtains, the branch road that ratio is big can be judged to the ground connection branch road; Survey along the ground connection branch road, when detecting the reflection electric current and diminish suddenly, corresponding sensing point is an earth fault; Said process successively carries out when searching.

2, detection method according to claim 1 is characterized in that: two low-frequency voltage signals that alternately apply, its voltage range are at 20～40V, frequency range f ₁Be 20～40Hz, f ₂Be 10～20Hz.

3, detection method according to claim 1 is characterized in that: the current signal that probe detects will pass through bandpass filtering, amplifies, and sends into microcomputer differentiation instrument (3) after the A/D analog-to-digital conversion process again and differentiates.

4, a kind of sniffer of making according to the described method of claim 1 is characterized in that: device is by the low-frequency voltage signal source (1) that can alternately produce two kinds of different frequencies, and portable probe (2), microcomputer are differentiated instrument (3) three parts and constituted.

5, sniffer according to claim 4 is characterized in that: described portable type current probe (2) is made of clamp on amperemeter (12), current compensation circuit (13) and amplifying circuit (14) series connection.

6, sniffer according to claim 4, it is characterized in that: there are two low-frequency voltage signal sources (4) and (5) in described low-frequency voltage signal source (1), (4) and (5) be parallel to the input end of automatic switch-over circuit (6), the output terminal of (6) is connected in series voltage and power amplification circuit (7), transformer and electric capacity output circuit (8) successively.

7, sniffer according to claim 4, it is characterized in that: the input signal hardware handles circuit that described microcomputer is differentiated instrument mainly comprises bandpass filter (16), (17), amplifier (18), (20), A/D converter (21), interface (22), (26), and computer part has single-chip microcomputer (23), program storage (24), data-carrier store (25).

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