KF-5 Pre-Location With Arc Process Eng

8/6/2019 KF-5 Pre-Location With Arc Process Eng

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Cable fault location in power cables

Pre-location with arc process


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Contents:

1. Introduction

2. ARM

3. ARMPlus4. DECAY Plus5. ARM- Burning

1. Introduction

More than 80% of cable faults are high-resistance faults. These faults generate no orvery slightly visible impedance changes and cannot be located with the classicimpulse reflection method.The classic conversion of a high-resistance to a low-resistance fault with a powerful

burner devices is used increasingly more rarely. Powerful burner devices are usedtoday for modifying the fault resistance in wet cables and for the pre-location inconnection with the ARMburner (arc burner). In the cable fault location using variouspre-location methods, the combination of a high-voltage process with the impulsereflection method has always been implemented and proved to be successful. For anarc burning on the fault location, the reflection factor is r = -1, because the faultresistance there is approximately zero, so almost represents a short circuit. Bycomparing a recorded reflection pattern without this burning arc (reference pattern)with a reflection pattern recorded with a constant arc on the fault position, it is possibleto determine the fault location. In the process, the two measured curves diverge at theposition of the ignited arc which corresponds to the fault location. In igniting an arc onthe fault location, essentially three basic principles are used.

1. Igniting the arc as a result of a sudden discharge of a charged capacitor in thecable. (application with all faults)

2. Igniting the arc by means of a DC voltage source as a result of charging thecable until breakdown (application with cables that can be charged)

3. Igniting the arc with a powerful DC voltage burner device (application withdamp cable faults)

To enable a defined triggering of the reflectometer it is necessary to stabilise the arc

and extend the arc burning period. When the measurement technician decides on aprocess it must still be checked whether a cable with a fault can actually be chargedand at which voltage it breaks down. Testing with DC voltage can establish thebreakdown voltage at the fault position. If, for example, the leakage current in thecable is too large, the cable is not able to be charged. In this case a process whichinvolves charging the cable cannot be used, high voltage impulse reflectionprocesses have to be chosen which work on the basis of discharging an impulsecapacitor. However, this decision is made in any case, because a cable that cannot becharged has a low ignition voltage which is the best condition for the ARM process.


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In principle, all devices which are used for the arc reflection process consist of thefollowing basic components.

1. A source of DC voltage (can also be the burning device)

2. An impulse generator, consisting of:a. an impulse capacitor

b. A switch which discharges the impulse capacitor to the measure-ment object

3. A filter which acts to extend the capacitor discharge which is necessary forstabilising the arc.

a. Inductively, a coil effects the arc extension

b. Resistively, a resistance delays the discharge of the capacitor

c. Actively, via another impulse unit with a lower voltage

4. A coupling unit which generates the necessary impulses for measurement itself,or couples the measurement pulses of the reflectometer to the high voltage.

Fig. 1: Simplified diagram of the pre-location of ARM

high voltage methods

The following processes for the measurement of HV arc reflection are available forselection in addition to other processes:

ARM for cable faults which can or cannot be loaded, short fault distances.Resistive as well as inductive methods are regarded as ARM processes

LSG 3E

ARMPlus for cable faults which can and cannot be charged, large fault distances

ARMBurner for cable faults which can and cannot be charged, damp faults

DECAY Plus for cable faults which can be charged up to 80 kV

High voltage sourceDC generator

Impulse generatorBurner device

Powerseparation

filter

Couplingunit

Reflectionmeasurement device

Teleflex MXTeleflex T30E

End of the cableCable fault

Measurement pulse

High voltageHigh voltage

Measured pulse

Reference pattern: without high voltageFault pattern: with high voltage


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Fig. 2: Reference and fault pattern

As a power cable rarely generates a clean, unhindered reflection pattern but apattern which always shows modifications, sleeves and other influences, a fault canonly rarely be detected by the user in a reflectorgram which is measured normally.

Therefore the ARM processes consist primarily of an OK and fault pattern. Only directcomparison allows the immediate and clear identification of the fault position. Bothmeasurements take place generally on the same wire. As the reference pattern ismeasured with lower voltage or as a normal reflectogram without high voltage, the faultis not visible there. Only the use of high voltage generates a clear negative reflection atthe fault position.

Exceptions to this are faults which appear in the OK pattern as negative reflection as aresult of their properties. In this case it is possible that the OK and fault patterns arealmost, or are completely identical. This fact, however, can be recognised in ameasurement cycle which is correctly carried out, by the fact that the fault resistance inthe insulation measurement is significantly below 100 Ohm.

Another problem is severed points. Various effects can occur in this situation.

a. The cable ends are so far separated from each other that there is no flashover.It is important here to verify that the visible end is actually the cable end! If the faultlocation shows that there could be a severed point, the far end should be earthed.This procedure would immediately be established on the reflectometer in the case ofan intact cable (see fault classification). If this earthing is not visible at the far end, i.e.there is no change of the polarity at the end, a severed point is most definitely theproblem. With impulses to the earthed end, a reverse behaviour is shown to thenormal parallel fault. (Frank, please create a reflection pattern with a severed point)The OK pattern shows an end, the fault pattern shows a longer cable with a smallreflection at the fault position but with a negative reflection at the real end.

Reference pattern without arc at the fault visible atthe end of the cable positive reflection

Fault pattern with arc at the faultFault visible negative reflection


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b. Another problem which could occur in the case of complete severed points is thebreakdown in one of the parallel wires which should be earthed with the correctconnection and establishment of the measuring arrangement. This means that themeasurement pulse may possibly not run to the end of the cable but via the parallelwire back to the earthed start of the cable. The resulting reflectogram in this case can

be very vexing owing to the completely unexpected course.

Above all, with such cable faults, it shows how important it is to following the generalrules and above all, the safety rules.

Whoever carries out the fault location in the correct sequence at the start will rarelybe in a situation in which the behaviour of the cable appears illogical andincomprehensible. An even more technical fault location will not be successful if theelementary rules of cable fault location are not followed.


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2. ARM(Arc Reflection Method)

ARMprocess (inductive)

The classic ARMprocess was patented by HDW in 1965. In this process, the discharge

of an impulse capacitor is carried out via an optimised series impedance for the ignitionof the arc on the fault position. After the decay of travelling waves which would disruptthe measurement, the decay process is carried out after a decaying sinusoidal vibrationwith a frequency of about 300 Hz (dependent on the test object system).Triggering the reflectometer and activating the measurement pulse takes place in thefirst current maximum of the decaying vibration. The measurement pulse isgenerated in the reflectometer. The maximum pulse amplitude that can be achievedis about 65 V with a pulse width of 5 s. This process is therefore particularly suitablefor the measurement of power cables with a total length of up to about 5 - 8 km.Positive results were also achieved with fault distances of up to 10 km.The measurement pulse from the reflectometer which is smaller in its amplitude hasthe advantage that the start region on the reflectogram is not covered by themeasurement pulse itself. The ARM process is therefore particularly well suited toshort cable lengths (up to 2 km) and small fault ignition voltages. The pulse width ofthe measurement pulse should not be below 500 ns for the first measurement. Withshort fault distances, the pulse width can be reduced in the second step.

Fig. 3: Simplified diagram for the ARM

process for arc stabilisation


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Double impulse processWith high impulse voltages over 12 or 16 kV, the double impulse is used to stabilisethe arc. (Centrix: 16/32 kV plus 4 kV, R30 system: 25/50 kV plus 12 kV impulsegenerator). With a double impulse process the fault is only first ignited with the high

voltage. However, the ionisation phase, which takes place during ignition, would notenable a stable pattern at such high voltages. Therefore, as soon as a sufficiently highand stable current is flowing, another impulse capacitor with the aforementioned lowervoltage in the arc is discharged and extends this arc significantly which then enables areliable measurement.


process for arc stabilisation by double impulses

HV impulse device

G

=

G

=

MV impulse device

Teleflex

ETF


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Fig. 5: ARM

process in a 8 km cable

Systems:

Surgeflex 8-1000Surgeflex 15/25 kVSPG 40/Compact city

Centrix 1 and 3R30 system

IndividualM 219

ARMprocess (active)A similar process to the ARM process and the double impulse process is the arcstabilisation. In this process an arc, which has previously been ignited with a highervoltage, is stabilised using an additional impulse capacitor and then measured with thereflectometer in the arc. The LSG 3E can be used, with its 2 kV impulse capacitor, alsodirectly as a pre-location device in the low voltage supply. The pulse width of themeasurement pulse should not be below 500 ns for the first measurement. With shortfault distances, the pulse width can be reduced in the second step.

Systems:Classic

IndividualLSG 3E


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ARMprocess (passive / resistive)

The simplest way to extend an arc is carried out using resistances. Which meansdischarging the impulse capacitor is extended by a resistance connected in series withtypically 300 Ohm. The method is called KLV (temporary arc process [Kurzzeit

Lichtbogen Verfahren]), or as SIM / S.I.M. (secondary impulse method). One of thefundamental disadvantages of this method is that a voltage which is discharged usinga resistance, is also always reduced. Consequently it is not always possible to reallytake the fault to breakdown with higher ignition voltage, or, that the previouslyestablished breakdown voltage of the fault is not necessarily the same as theperformance parameters of the impulse generator used. Advantages include the handysize, weight and the favourable price of such a simple filter. As a rule, severalmeasurements have to be taken. Storing the individual measurements is alwaysrecommended, this is carried out automatically when using the Teleflex MX. The pulsewidth of the measurement pulse should not be below 500 ns for the first measurement.With short fault distances, the pulse width can be reduced in the second step.

Systems:Surgeflex 32 kVClassic (optional)

IndividualLSG 300


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3. ARMPlus process

The ARM Plus process is a process with active arc stabilisation for cable faults up toa maximum impulse voltage of 32 kV. It is based on the production of a high voltage

measurement pulse for the running time measurement. From 16 kV using the so-calleddouble impulse principle, a stable arc with an adequately long burning period is ignitedin the fault position. The actual stabilisation of the arc is achieved by the coupling ofanother impulse capacitor level (4 kV) which is charged to a defined value. The arcburning period is dependent on the oscillating circuit parameters resulting from thealignment of the test object - test system, and the insulation which is dependent on thecable length, and lies in the range of a few ms. The measurement pulse for the faultlocation arises from the impulse discharge of a pulse capacitor via spark gaps and hasa maximum pulse height of 1500 V. The measurement pulses which are very energeticin comparison enable a fault location in power cables with up to approx. 10 km length.Coupling out of the signals on the reflectometer is carried out by a Rogowski coil in

the low end of the pulse capacitor.The fault position is detected by the divergence of the two curves (between the OKand fault pattern) at the fault position.

Fig. 6: Simplified diagram for the ARM plus process for arc stabilisation

DC supply

G

=

HV impulse device16 / 32 kV

MV impulse device4 / 8 kV

1 kV 200 V

Teleflex


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Fig. 7: ARM

plus process in a 8 km cable

Systems:

Centrix 1Centrix 3


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4. Decay Plus process

The Decay Plus process allows the fault pre-location in chargeable cables with veryhigh ignition voltages of up to 80 kV. It extends the ARM Plus process, which is

limited by the maximum charging voltage of the impulse capacitors by the amount ofthe fault ignition voltage, to the test voltage limit of 80 kV.The arc is ignited on a closed working spark gap by charging the cable until theflashover at the fault position. The actual extension of the arc is achieved by thecoupling of a lower impulse capacitor level (4 kV) which is charged to a defined value.A stable burning arc is obtained at the fault position which is used as the reflectionlevel for the measurement pulse. The measurement pulse also arises in this case bythe discharge of a pulse capacitor using spark gaps with a maximum pulse height of1,500 V. With this process as well the measurement pulse, which is also veryenergetic, enables a fault pre-location with power cables of up to approx. 10 km long.The fault position is detected by the divergence of the two curves (between OK and

fault pattern) at the position of the fault and has, in principle, an identical course incomparison with the ARMPlus process.

Fig. 8: Simplified diagram for the Decay Plus process for arc stabilisation

Systems:Centrix 1Centrix 3

DC generatorup to 80 kV

Impulse device1 kV 200 V

Teleflex

G

=


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Fig. 9: DECAY Plus,Reference and fault pattern


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5. ARM Burning

Despite all of the other available technologies, burning has not completely disappearedfrom the process of cable fault location. Special wet sleeves and similar problemsquickly bring the majority of other methods to their physical limits. To design a burning

process that is as simple and effective as possible, ARM

and burning were combined,i.e. during the burning process, a continuous arc reflection measurement also takesplace. In doing so this technique allows the tracing of the modification of the fault onthe display screen. The fault distance can be measured immediately and the systemstops the burning process automatically as soon as a stable low resistance stateis achieved. As with all the other arc processes a reference and fault pattern arethen compared.An additional pre-location is not necessary, the process can go directly to thepinpointing from the burning.The advantage of this method compared with conventional burning is the controlledprocedure which restricts the actual burning to the shortest necessary time. As a

result, this burning takes place as quickly and as gentle on the cable as possible.


burning process

SystemsCentrix 1Centrix 3R30 system

R

G

=

Burner device

Teleflex

ETF


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In addition to the arc reflection process, there are also the so-called transientprocesses of current and voltage coupling.

These processes as well as their various application possibilities will be described in

one of the next reports.

Documents

KF-5 Pre-Location With Arc Process Eng