IE921252A1

IE921252A1 - A method for determining an earth-shorted branch circuit in¹an electrical supply or distributor network

Info

Publication number: IE921252A1
Application number: IE125292A
Authority: IE
Inventors: Elektro-Bau Ag
Original assignee: Elektro Bau Ag
Priority date: 1991-04-19
Filing date: 1992-04-16
Publication date: 1992-10-21
Also published as: WO1992018872A1

Abstract

In a process for detecting a branch circuit (14A, 14B, ...) subject to earth leaks in an electric power supply or distribution system, in which an earth leak quenching choke (2) is inserted between the mains neutral point and earth, with mains operation switched off, the difference for each individual circuit (14A, 14B, 14C) of the admittances resulting as the quotient of the branch zero current and the neutral point earth voltage is determined before and after each change in the neutral point impedance and the circuit in which this difference is at its maximum is taken as the one subject to earth leaks. The quench coil is plunger choke coil (2) in which the plunger setting is altered between the measurements. The admittances of the individual circuits (14A, 14B, 14C) are calculated from measurements of the branch zero currents as the sum of the three branch phase currents (e.g. with a Holmgreen circuit) and the biasing voltage between the neutral point and earth, taking account of the phase relationship between the zero currents and biasing voltage and complexly subtracted for each individual branch.

Description

The invention relates to a method for determining an earth-shorted branch circuit in an electrical supply or distributor network, in which an earth-short inductor is connected between the network neutral point or trana5 former neutral point and earth, the difference in the conductances which are produced ae quotients of branch zero current and offset voltage in magnitude and phase being determined for each individual circuit before and after a change in the neutral point impedance when network operation is deactivated, and that circuit at which this difference is at a maximum, that is to say is approximately unequal to zero, being evaluated as earthshorted.

In conparison to other known methods which provide separate earth-short direction relays and earthshort intermediate relays for the individual branch circuits, a method of this kind has the advantage that it only requires a small number of measurement points, fault location still being possible as a result of high earth contact resistances even in the case of earthshorts with only small offset voltages. The method can be principally need in radial networks, in the case of changes in the neutral point Impedance, no changes in the abovementioned conductance occur in fault-free branches, whereas in an earth-shorted branch the conductance experiences a change due to the leakage away to earth so that the difference in the conductances before and after the change in the neutral point iopedance is unequal to zero.

The objeat of the invention ie to specify a method which permits an earth-shorted branch circuit to be determined when required automatically and with sufficient sensitivity using single means and, above all, without changes ia tha network or the earth-short inductor in the power component.

The object set is achieved in that a plunger inductor is used as the inductor and its plunger setting ie varied between measurements, the conductances of the - 2 individual branch circuit· being calculated from measurement· of the branch zero current a· the sum of the phase currents flowing in the branch and of the offset voltage between the neutral point and earth taking into account the phase position between zero current and offset voltage and being subtracted in complex fashion for each individual branch and that branch at which this difference is at a ziaximom being evaluated ae earthshort ed. λ plunger inductor has, per se, the advantage that it can be adjusted when an earth-short occurs or in the vicinity of a resonance point, at which resonance prevails in an imaginary equivalent circuit of a parallel resonant circuit formed from a coil inductor, the sum of the ohmic tap-offs and the sum capacitance, in order to promote elimination of the earth short. In the method according to the invention, it is sufficient to have simple measurement tap-offs and a computer in order to be able to carry out the necessary determinations without costly additional devices, in particular in the power component of the network or of the inductor. Normally, the plunger inductor is essentially set to resonance during the first measurement and ia detuned from this setting by a predetermined amount for the second measure25 ment cycle.

However, theoretically and practically the method according to the invention can even be carried out if only the condition is fulfilled that the two measurement points correspond to different coil positions. It is possible to control the drive device of the plunger inductor via the computer in order to detune the inductor after the first measurement cycle and to perform resetting or adjustment to the resonance point after the second measurement cycle, it being possible to determine this resonanoe point in a fashion which supplements the described method by approaching a third plunger position.

Further details and advantages of the subject of the invention can be taken from the subsequent description of the drawing.

In the drawing, the subject of the invention ie illustrated by vay of example; in the drawing» Fig.l shove a block diagram of a measuring system which can be used to carry out the method according to the invention and Fig. 2 shows an equivalent circuit for an earth-shorted branch circuit which is connected to the earthshort inductor, for the purpose of explaining the method according to the invention.

Fig. 1 shows a diagrammatic illustration of an equipannt array for determining an earth-shorted branch in a radial network. The neutral point of the secondary side cf a network transformer 1 is connected to a plunger inductor 2, the plunger of which can be adjusted with the aid of a drive device 3. The offset voltage between neutral point and earth which is supplied to the inductor 3 is detected by means of a voltage transformer 4 and transformed and subsequently matched in a further trans20 former stage with a band-pass filter 5 and freed from harmonics. In a signal preprocessing unit fi, the effective value of the measurement variable ia determined and subsequently converted into digital form with the aid of an analog-to-digital converter and passed on to a computer 12. In addition, at β a square-wave signal which is in identical phase with the measurement variable ia tapped off for further digital processing and passed on to the computer 12.

In all the branches 14A, 14B, 14C .... of the network 14, the zero currents associated with the respective- branches are determined via composite current transformers 7A, 7B, 7C ..., e.g. in a Holmgreen circuit, and subsequently converted via a DC isolating current/voltage converter BA, 8B, 8C .... into a form which can be electronically evaluated. A signal multiplexer 9 which is controlled by the computer 12 switches the zero current signal which can ba selected by the computer 12 to a band-pass filter 10 where it is - 4 freed free harmonics end fed to e signal preprocessing unit 11 in which the effective value of the selected, filtered zero current signal is formed and converted into a digital fora via an onalog-to-digital converter. A square wave signal which is in identical phase with the measurement variable is also formed in the signal preprocessing unit 11 and passed on to the ooaputer 12 just like the digital value of the effective value of the measurement variable.

In the event of an earth-short, at a coil position located in the vicinity of the resonance tuning all the transformers SA, 8B, 8C etc. which are associated with the branches 14A, 14B, 14C... are selected by means of the eignal multiplexer 9 and the measurement data received axe detected for the sero currents* At the same time, measurement data of the neutral point earth voltage are detected via 4, 5, 6 and fed to tbe computer. In tbe computer 12, the quotients are calculated for all tbe branches 14A, 14B, 14C from the effective values of the sere currents and the effective value of the momentary offset voltage and the phase shift of the respective zero current with respect to the offset voltage is also determined and the conductances which are thus determined in a complex manner by means of magnitude and phase for each branch are stored. Subsequently, the plunger of the plunger coil 2 is adjusted via the drive device 3 with the effect of detuning, it being possible to control this adjustment also from the computer 12. Subsequently, the measurements carried out in the first position of the plunger are repeated end the calculation of the complex conductances performed, the corresponding values being stored again. Subsequently, the conductances determined during the first measurement cycle (before the plunger coil adjustment) are subtracted in complex fashion for each branch 14A, 14B, 14C... from the conductances determined in the second measurement cycle (after the plunger coil adjustment), That branch at which the magnitude of the difference between the two conductances - a is at « maxioum ie detected as earth-shorted and aorreepcnding storage is performed for it. Now, the entire process is repeated. If the sabs branch is identified once more as earth-short ed, the result is indicated on a display unit 13. Otherwise# the measurement process is repeated until the same branch has been identified as earth-shorted during two successive measurement processes.

In the circuit diagram according to Fig. 2, an 10 equivalent circuit is illustrated for a branch circuit, e.g* 14A. The plunger coil 3 ie located at the neutral point of the transformer 1. The three phases of this branch oirouit have phase voltages which can be measured in relation to earth. In the oirouit, the conductances of IS the ohmic tap-offs of the individual phases with respect to earth were designated by g^, g^, gi, and the capacitance of the individual phases with respect to earth are designated by Cg, Cg, C,. In the case of an earth-short which has been assumed for the top phase, this short has with respect to earth a conductance of the earth contact resistance. In the branch, a zero current ie flows which is determined for example with the aid of a Bolmgreen circuit and an offset voltage ie applied again to the inductor 3. In the presence of an earth-short, the conductance of the branch whose magnitude and phase is determined as a quotient of the zero current and offset voltage changes at the two settings of the coil 3 whenever the earth contact resistance has a very high Impedance so that the branch can be identified ae earth-shorted.

Claims

1. Patent Claim

1. Method for determining an earth-shorted branch aircuit in an electrical supply or distributor network in which an earth-short inductor ie connected between the 5 transformer neutral point and earth, the difference in the conductances which are produced as quotients of the branch zero current and offset voltage in magnitude and phase being determined for each individual aircuit before and after a change in the neutral point impedance when 10 network operation is deactivated, and that circuit at which this difference is at a maximum being evaluated as earth-shorted, characterised in that a plunger inductor (3) is used es the inductor and its plunger setting is varied between the measurements, the conductances of the 15 individual branch circuits (14, 14A, 14B, 14C....··) being calculated from measurements of the branch zero current ae the sum of the phase currents flowing in the branch and of the offset voltage between neutral point and earth taking into account the phase position between 20 zero current and offset voltage and being subtracted in complex fashion for each individual branch and that branch at which this difference is unequal to zero being evaluated as earth-shorted.

2. A method according to claim 1 for determining an earth-shorted branch circuit in an electrical supply or distributor network, substantially as hereinbefore described with reference to the accompanying drawings.