In electric power distribution systems, a wide variety of
cable faults can occur. The problem may be in a communication circuit or in a
power circuit, either in the low- or medium-voltage class.
Regardless of the class of equipment involved or the type of
fault, the one common problem is to determine the location of the fault so that
repairs can be made.
The vast majority of cable faults encountered in an electric
power distribution system occur between conductor and ground. Most
fault-locating techniques are made with the circuit deenergized.
In ungrounded or high-resistance grounded, low-voltage systems,
however, the occurrence of a single line-to-ground fault will not result in
automatic circuit interruption; therefore, the process of locating the fault
may be carried out by special procedures with the circuit energized.
Once a line-to-ground fault has occurred, the resistance of
the fault path can range from almost zero up to millions of ohms. The fault
resistance has a bearing on the method used to locate the failure.
In general, a low-resistance fault can be located more
readily than one of high resistance. In some cases, the fault resistance can be
reduced by the application of voltage sufficiently high to cause the fault to
break down as the excessive current causes the insulation to carbonize.
A wide variety of commercially available equipment and a
number of different approaches can be used to locate cable faults. The method used to locate a cable fault
depends on the following:
a) Nature of fault
b) Type and voltage rating of cable
c) Value of rapid location of faults
d) Frequency of faults
e) Experience and capability of personnel
Megohmmeter Instrument Test
When the fault resistance is sufÞciently low that it can be
detected with a megohmmeter, the cable can be sectionalized and each section
tested to determine which contains the fault. This procedure may require that
the cable be opened in a number of locations before the fault is isolated to
one replaceable section. This could, therefore, involve considerable time and
expense, and might result in additional splices. Since splices are often the
weakest part of a cable circuit, this method of fault locating may introduce
additional failures at a subsequent time.
Conductor Resistance Measurement
This method consists of measuring the resistance of the
conductor from the test location to the point of fault by using either the
Varley loop or the Murray loop test. Once the resistance of the conductor to
the point of fault has been measured, it can be translated into distance by
using handbook values of resistance per unit length for the size and conductor
material involved, correcting for temperature as required.
Capacitor Discharge
This method consists of applying a high-voltage and
high-current impulse to the faulted cable. A high-voltage capacitor is charged
by a relatively low-current capacity source such as that used for
high-potential testing. The capacitor is then discharged across an air gap or
by a timed closing contact into the cable. The repeated discharging of the
capacitor provides a periodic pulsing of the faulted cable. The maximum impulse
voltage should not exceed 50% of the allowable dc cable test voltage since
voltage doubling can occur at open circuit ends. Where the cable is accessible,
or the fault is located at an accessible position, the fault may be located
simply by sound.
Tone Signal
A tone signal may be used on energized circuits. A
Þxed-frequency signal, generally in the audio frequency range, is imposed on
the faulted cable. The cable route is then traced by means of a detector, which
consists of a pickup coil, receiver, and a head set or visual display, to the
point where the signal leaves the conductor and enters the ground return path.
This class of equipment has its primary application in the
low-voltage Þeld and is frequently used for fault location on energized
ungrounded circuits. On systems over 600 V, the use of a tone signal for fault
location is generally unsatisfactory because of the relatively large
capacitance of the cable circuit.
Radar System
A short-duration, low-energy pulse is imposed on the faulted
cable and the time required for propagation to and return from the point of
fault is monitored on an oscilloscope. The time is then translated into
distance to locate the fault. Although this equipment has been available for a
number of years, its major application in the power field has been on
long-distance, high-voltage lines.
No comments:
Post a Comment