Cable Fault Tone Traccing

TONE TRACING: EQUIPMENT CONSTRUCTION, OPERATIONAL
PRINCIPLES, AND BASIC LOCALIZATION TECHNIQUES

Equipment Construction

Tone tracing equipment includes a tone generator and a signal tracer. Tone Generator A tone generator, sometimes called a transmitter, is illustrated in Figure 1. A tone generator is a suitcase-size test set that is powered by 120 volts. It has controls that adjust the magnitude of high-voltage DC output and
the frequency of its oscillator output

Signal Tracer

Figure 1 also contains an illustration of a signal tracer. The signal tracer consists of a hand-held detector and an insulated-shaft probe. The signal tracer is sensitive to those frequencies that the tone generator produces, and it filters all other audio-frequency signals.
 

Equipment Operational Principles

Tone Generator

The tone generator has a built-in sine-wave oscillator that injects an audio-frequency current into the core conductor of a faulted cable. This audio frequency current is driven by a solid-state amplifier that is typically rated at 2.5 watts of output power. The output is usually a single frequency, but some models have more than one selectable frequency. Audio frequencies are in the range of 10 Hz to 10,000 Hz depending on the specific model and manufacturer of the tone generator.

The tone generator also has a built-in high-voltage DC source that can be used to break down a high-resistance shunt fault for the purpose of providing a low-resistance path for the audio-frequency current.

Signal Tracer

The signal tracer has an inductive pickup built into its probe. The output signal of this probe and the sound it he headphones become stronger as the probe is brought closer to a cable.
Basic Localization Techniques Methods

The basic method of locating a shunt fault is to adjust the high-voltage DC output of the tone generator to a magnitude that will make the damaged insulation in the cable flash over. The audio-frequency current allows the technician to trace the path of the buried cable. The location of the fault is found by sweeping the probe over the surface of the earth above the cable and listening for the characteristic sound of flashover in the headphones.

The basic method of locating an open-circuit fault is to adjust the high-voltage DC output to a minimum and adjust the audio-frequency current to a maximum. The location of the fault is found by sweeping the probe over the surface of the earth above the cable and listening for the audio tone.

Maximum Signal - For a shunt fault, the characteristic sound of a flashover will be loudest in the detector’s headphones when the probe is oriented directly above the fault.

For an open circuit fault, the audio tone heard in the detector’s headphones will fade and become inaudible when the technician sweeps the probe past the location of the open circuit.

Null Signal - The probe’s inductive pickup has a directional characteristic such that whenever the probe is oriented in parallel with a cable that is carrying an audio tone, its output signal strength will become near zero (null signal). When the probe is oriented perpendicular to a cable carrying an audio tone, its output signal will reach a maximum

Minimizing Interference and Crosstalk

Electromagnetic interference is produced by any cable that is energized with normal voltage. In order to minimize interference, nearby cables should be de-energized to whatever extent is possible without disrupting electrical service.

The audio-frequency current that is intentionally injected into one power cable can unintentionally induce an audio-frequency current in another cable that is buried nearby. This unintentional induced current, called crosstalk, can mislead the technician who is tracing the path of a cable. In order to minimize crosstalk, deenergized cables should be connected to ground on both ends.

Cable Earth-Gradient Detector Auxiliary Device

Figure 1 is an illustration of an earth-gradient detector. 

This detector has two spikes that are driven into the earth above a buried cable. The current that flows through the earth in the vicinity of a cable when the cable’s insulation breaks down under the stress of the thumper’s high-voltage pulse causes a difference in potential
between these spikes. When connected to the spikes, the microammeter of the detector deflects to the left or to the right according to the direction of the current flowing from one spike to the other.

Earth-Gradient Localization

Figures 2a and 2b illustrate the basic method of using an earth-gradient detector to locate a fault in a buried cable. Current flows in several paths through the earth from the point of the damaged insulation to the driven rod. These paths are represented by broken-line curves in Figure 7. These currents produce a voltage gradient between any two points at the surface of the earth. The technician locates the fault by placing the spikes of the earth-gradient detector at different surface locations along a straight line between the ends of the cable. At successive locations (1, 2, and 3) the technician reads the deflection of the detector until he reads a reversal of deflection (location 4). The technician backtracks until he finds a location (5) where there is a null deflection.

The technician then reads deflections (6, 7, and 8) along a line that crosses the first line at a right angle. The location of the fault is at the second null deflection (location 8). Through the use of this method, the technician does not need to know the route of the buried cable in order to locate the cable fault.

                     

Cable Acoustical Detector Auxiliary Device

Figure 1 is an illustration of an acoustical detector, an auxiliary device that is used with both types of thumpers. An acoustical detector has two sound transducers that are placed on the ground above a buried cable. The sound made by the cable when its insulation breaks down under the stress of the thumper’s high-voltage pulse causes an upscale deflection of the detector’s output level meter. A set of headphones can also be plugged into the detector so that the amplified and filtered sound of the breakdown can be heard.

Acoustical Localization
Figure 2 illustrates the basic method of using an acoustical detector to locate a fault in a buried cable. The pulses transmitted by the cable thumper cause the damaged insulation of the cable to break down repeatedly.

Each breakdown produces a sound. This sound can sometimes be heard above the ground. But for those cases when the sound is not loud enough to be heard, an acoustical detector is used to locate the damaged insulation.

The acoustical detector is able to distinguish the relative intensity and time delay between the arrival of the thump sound at its two pickups. The technician moves the location of the acoustical pickups until the thump sound is equal in intensity in the two earpieces of the headphones. The location of the fault is then directly below and midway between the pickups.

CABLE THUMPING

CABLE THUMPING: EQUIPMENT CONSTRUCTIONS, OPERATIONAL
PRINCIPLES, AND BASIC LOCALIZATION TECHNIQUES

A cable thumper is an electrical test set that generates repetitive high-voltage high-energy pulses. A cable thumper transmits these pulses into a power cable in order to cause a fault in the cable to break down and, consequently, produce an audible sound and a strong current in the earth surrounding the fault. The sound reveals the location of the fault. If the sound is not easily heard at the surface of the earth, an acoustical detector is used to locate the cable fault. Alternatively, an earth-gradient detector can be used to locate the fault by sensing the earth currents that flow near the fault.
 

Thumper Constructions

There are two basic types of cable thumpers: the series-gap type and the pulse type. There are two basic types of detectors: the acoustical detector and the earth-gradient detector. 

Either type of detector can be used with either type of thumper. The constructions of thumpers and detectors are explained in the next four subject headings.

                          Illustration of a Cable Thumper

Series-Gap Type
 

Figure 1 is an illustration of a series-gap type of cable thumper. The illustration shows the following:
· A knob-controlled variable transformer. This transformer controls the magnitude of high-voltage output
pulses.
· A kilovoltmeter. This meter indicates the voltage of the thumper’s built-in impulse capacitor.
· A primary ammeter that indicates the input current.
· A microammeter that indicates the output current.
· A power cord.
· An output test lead.
· Jacks for connecting the battery leads.
· An impulse-control gap handle. This handle adjustments the dimension of the series gap.

Pulse Type

 
Pulse-type cable thumpers have the same general construction as series-gap cable thumpers. The important difference in construction is that a set of additional components allow the rate that output pulses are generated to be adjusted independently from the output voltage adjustment.


Thumper Operational Principles

Generating a High Energy Pulse in a Series-Gap Thumper
Figure 2a is a simplified schematic diagram of a series-gap thumper. The thumper’s high-voltage power supply is similar to the power supply of a DC applied potential test set. This power supply charges an impulse capacitor. A kilovoltmeter indicates the magnitude of the impulse capacitor’s voltage. A variable transformer is used to control the maximum voltage that charges the impulse capacitor.

Before a test, the series gap is adjusted to a maximum dimension. The voltage of the capacitor is adjusted to the level that is appropriate for testing the cable, and the dimension of the series gap is subsequently adjusted until it flashes over. This flashover causes a pulse of high voltage to be transmitted into the cable under test and also discharges the capacitor. A short interval of time (approximately one to 30 seconds) elapses before the capacitor charges to a voltage level high enough to again cause the series gap to flash over. Pulses of high voltage are repeatedly transmitted into the cable. The interval of time between pulses can be shortened by adjusting the gap to a smaller dimension. Making the gap smaller consequently reduces the peak voltage magnitude of the output pulses.

Generating a High Energy Pulse in a Pulse-Type Thumper

Figure 2b is a simplified schematic diagram of a pulse-type thumper. The operational principle of a pulse-type thumper is the same as that of a series-gap thumper except that the time interval between pulses is controlled by a high-voltage contactor and a timing circuit. For a pulse-type thumper, the time interval between pulses can be adjusted without affecting the voltage magnitude of the output pulses.

Electrical Cables Go, No-Go Overpotential Test

The hi-pot test can be conducted as a go, no-go overpotential test. In this test the voltage is gradually applied to the specified value. The rate of rise of the test voltage is maintained to provide a steady leakage current until final test voltage is reached. Usually, 1–1.5 min is considered sufficient for reaching the final test voltage. The final test voltage can then be held for 5 min, and if there is no abrupt increase in current sufficient to trip the test set, the test has been successfully passed. This test does not provide a thorough analysis of cable condition, but provides sufficient information as to whether the cable meets a specific high-voltage breakdown strength requirement. This type of test is usually performed after installation and repair, where only cable that can withstand strength verification without a breakdown is to be certified.

Voltage versus Leakage Current Test (Step-Voltage Test)

In this test, the voltage is raised in equal steps and time is allowed between each step for leakage current to become stable.the current is relatively high as a voltage is applied owing to capacitance
charging current and dielectric absorption currents. As time passes, these transient currents become minimum with the steady-state current remaining, which is the actual leakage current and a very small amount of absorption current. At each step of voltage, the leakage current reading is taken before proceeding to the next step. Usually, it is recommended that at least eight equal steps of voltage be used and at least 1–4 min be allowed between each step. The leakage current versus voltage are then plotted as a curve. As long as this plotted curve is linear for each step, the insulation system is in good condition. At some value of step voltage, if the leakage current begins to increase noticeably, an increase in the slope of the curve will be noticed, as shown in Figure 1.1. If the test is continued beyond this test voltage, the leakage current will increase even more rapidly and immediate breakdown may occur in the cable insulation. Unless breakdown is desired, the test should be stopped as soon as the increase of slope is noticed in the voltage versus leakage current curve.
Maximum leakage current allowable for new cables acceptance can be determined from the ICEA formula for minimum allowable insulation

   
       

        

Figure 1.1 Step-voltage hi-pot test current.

resistance discussed earlier. The formula for leakage current then can be written as follows:

     

where
IL is the conduction or leakage current
E is the test voltage impressed
K is the specific insulation resistance megohms per 1000 ft at 60°F
D is the diameter over insulation
d is the diameter over conductor

The typical specific insulation resistance (K) for various commonly used insulations for cables are given under discussion of insulation resistance measurement test.
In order to explain the use of this formula, an example is given below for determining the maximum leakage current allowable for a 15 kV, 500 kcmil cable for an acceptance test.

Example
A 15 kV cable 500 MCM 220 Mil XLPE insulation conductor OD = 0.813 Class B strand. The circuit is 2500 ft long. Calculate the maximum leakage current at maximum test voltage of 65 kV.

   

Electrical Cables and Accessories Testing 2

DC Overpotential Testing
 
In the past, this test has been extensively used for acceptance and
maintenance of cables. Recent studies of cable failures indicate that the DC overpotential test may be causing more damage to some cable insulation, such as cross-link polyethylene, than the benefit obtained from such testing. It can indicate the relative condition of the insulation at voltages above or near operating levels. This test can be used for identifi cation of weakness in the cable insulation and can also be used to break down an incipient fault. A typical DC test set is shown in Figure 1.1. Generally, it is not recommended that this test be used for breakdown of incipient faults even though some test engineers use it for this purpose. Therefore, the incipient fault breakdown probability should be anticipated before and during the hi-pot test. The impending cable failure will usually be indicated by sudden changes in the leakage current, and before insulation is damaged, the test can be stopped. The test voltage values for DC hi-pot tests are based upon fi nal factory test voltage, which is determined by the type and thickness of insulation, the size of conductors, the construction of cable, and applicable industry standards. The DC test values corresponding to AC factory proof test voltages specified by the industry standards are usually expressed in terms of the ratio of DC to AC voltage for each insulation system. This ratio is designated as K, which when multiplied by the acceptance test factor of 80% and maintenance factor of 60% yields the conversion factors to obtain the DC test voltages for hi-pot tests. These recommended test voltage conversion factors are shown in Table 1.1. Also, the IEEE standard 400.1–2007 lists the voltage values for conducting hi-pot acceptance and maintenance tests in the field for laminated shielded power cables, which are shown in Table 1.2.

Many factors should be considered in selecting the right voltage for existing cables that are in service. As a general rule, for existing cables, the highest values for maintenance should not exceed 60% of final factory test voltage,

 

Figure 1.1 
DC hi-pot test set, 70 kV. (Courtesy of Megger, Inc., Valley Forge, PA.) 

and the minimum test value should be not less than the DC equivalent of the AC operating voltage. If the cable cannot be disconnected from all the connected equipment, the test voltage should be reduced to the voltage level of the lowest rated equipment connected. The hi-pot test can be conducted as a step-voltage test as discussed next.

Table 1.1 Conversion Factors for DC Hi-Pot Tests


   
     
 Table 1.2
Field Test Voltages for Laminated Shielded Cables up to 69 kV System Voltage
  
Note: Voltages higher those listed, up to 80% of system BIL for installation and maintenance testing may be considered in consultation with the suppliers of cable and the accessories.
When equipment, such as transformers, motors, etc., is connected to the cable circuit undergoing a test, voltages lower than recommended values may be used to comply with the limitations imposed by the connected equipment. a Maintained for a duration of 15 min.