basic tdr techniques paper 2010-5-13[1]

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Hipotronics, IncP.O. Box 414, 1650 Route 2

Brewster, NY 10509-041Tel: (845) 279-364

Fax: (845) 279-246Test Bus iness High Vo l tage

Basic TDR Fault Locating Techniques

Abstract:

When under ground cables fail it can a very difficult, time consuming, and an expensive process to repair

or replace the cable. This paper discusses how Time Domain Reflection (TDR) can help speed up and

reduce costs in replacing underground cable. TDR fault locating uses radar technology to help determine

where the failures or imperfections are in an underground cable system. This is done performing

calculations, which include velocity of propagation and reflection coefficients, on cables that are

characterized by a simple RLC network. Failures in underground cable can include water in the cable,

changes in insulation and concentric neutral corrosion. By using the three stake method and a TDR users

can typical determine the location of the fault within 1% accuracy of the total cable length.

General:

TDR method can be compared to Radar. Suitably shaped electrical pulses are transmitted thorough the

cable under test. Imperfections in the cable being tested cause reflections. By examining the shape, size

and time delay of these reflections, it is possible to determine the nature and location of an imperfection,

which may be caused by a splice, cable transition, transformer, short, open, fault, etc.

Transmit Pulse:

Size and shapes are described as Pulse Width and Pulse Amplitude. Pulses used in TDR’s are very narrow.

Most systems start as low as 50nS for short lengths ranging up to 5 or 10uS for long lengths. Internal pulse

amplitude normally ranges between 15 to 20 volts with minimal amount of energy which poses no danger

to the operator and does not cause any damage to the cable under test.

Characteristic Impedance:

Cable impedance designs can be described as a distributed parameter of an electric network. The network

design of a power cable is shown in figure 1.

• L= Series inductance of stored energy in the magnetic field per unit length

• C= Shunt capacitance of stored energy in the electric field per unit length

• R= Series resistance per unit length

• G= Losses in the dielectric by the shunt resistance per unit length

• Z0= Combined parameter characteristic impedance

Z0 for power cables has a value between 10 – 75 ohms.

Fig. 1 - Distributed – parameter electric network

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Terms used in TDR Fault Locating:

Zero Point, The zero point is taken at the end of the TDR or HV Coupler test lead. When this point is

defined the zero point the TDR will automatically deduct the length of the test leads from the measured

distance.

Memory, The TDR has a set number of memory locations that allow waveforms to be saved for futurereference if required. By selecting the appropriate memory location, and the waveform and corresponding

settings will be saved.

Downloading, The TDR software package allows stored waveforms in the TDR to be downloaded to a PC

for archiving and future analysis.

Range or Pulse Width, When using a TDR, it is important to get as close as possible to the fault you are

trying to locate. Once the test location has been chosen, a suitable range or pulse width must be selected on

the TDR.

Balance, Most TDR”s have balance controls, which is used for matching the impedance of the TDR to the

cable under test. An impedance mismatch causes a reflection pulse to be shown at the start of the TDR

trace.

Gain, The gain on the TDR should be adjusted so that the fault pulse can be clearly seen, but not so much

that it is saturating on the screen. Ideally it should be about halfway to three quarters up the display.

Velocity Factor, The VFR should be set according to the cable under test. If the exact value is unknown, a

value can be chosen from the tables supplied by the TDR manufacturers.

Cursor, The cursor is positioned at the start of the fault pulse at the point the curve breaks form the

horizontal. This point is the start of the fault and is the distance form the TDR to the fault.

Fault Types and Reflections:

Transmitted pulses generated by the TDR travels along a cable at a certain speed, called the velocity of propagation. With no changes in the cable i.e. fault free, no splices, the pulse continues down the cable

with gradual attenuation. With an irregularity in the cable i.e. cable fault, splices will cause a reflection to

occur on the TDR screen as the pulse travels down the cable. If an open or a short is detected then 100% of

the pulse will be deflected at that point. “Opens cause a 100% positive reflection and shorts causes a 100%

negative reflection to occur.

Transmit Pulses

Fig. 2 - TDR pulse reflections for an open and short circuit.

Other variations on these pulses are displayed in figure 3, 4, and 5

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Fig 3 - TDR pulses typical found in underground cable system

Fig. 4 - Other typical TDR Pulses

Fig. 5 - TDR pulses with problematic underground cable

Velocity of Propagation

Different types of cables have an effect on how fast the TDR pulse travels down the length of cable. For

accurate distance measurements it is important to look at the speed with which the pulse travels on a

particular type of cable. This speed is referred to as velocity of propagation (typical values seen in Table 1on the next page).

Rather than expressing the Velocity of Propagation, (Vp ) in miles per hour it generally is expressed in feet,

yards, or meters per microsecond (µs). This Velocity of Propagation value varies from one type of cable to

another and remains pretty constant for any one type of cable. This value of Vp for any particular cable

depends on the dielectric constant of the insulating material used.

In TDR measurements Vp /2 is used. Taking ½ of Vp is done to eliminate confusion over the fact that

reflection cannot be observed where and when it actually occurs. The pulse has to travel to the fault and

back to the test set, which then will be displayed on the LCD screen of the TDR. This measurement would

be twice the distance we are seeking. Using Vp /2 instead of Vp, the distance indicated on the LCD screen

of the TDR is the true distance to the fault.

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Typical velocity of propagation values

KV class Mils Cable Size Type Desc. Vp/2

15 175 #1CU XLPE 277

15 175 #2AL XLPE 248

15 175 #2CU XLPE 257

15 175 #4CU XLPE 258

15 175 1/O XLPE 261

15 175 1/O TRXLPE 255

15 175 2/O XLPE 243

15 175 4/O XLPE 236

15 175 500MCM XLPE 263

15 175 750MCM XLPE 268

15 175 1000MCM TRXLPE 215

15 175 #2AL EPR 272

15 175 1/O EPR 254

15 175 4/O EPR 286

15 220 1/OAL TRXLPE 275

25 260 1/OAL TRXLPE 27025 260 1/OAL TRXLPE CU Ribbon 285

35 345 1/O XLPE 283

-- 2/O Mining Trailing 215

4/OMPF 270

2/OMPF 260

2/O GGC Round 250

#2 GGC Round 270

#4-4/c W Round 265

Pilot Wire Avg. 300

* When sectionalizing values will vary depending on the number of

transformers and splices* A typcial Vp/2 number to use on power cable is between 250 to 260

Table 1 – Typical velocity of propagation values for various cable types

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Reflection Coefficient

The magnitude of the pulse reflections depends on how much the fault impedance; Z differs from the

characteristic impedance, Z0. The amount of reflection, r can be calculated from:

r = Z – Z0Z + Z0

For a short circuit Z = 0 and the reflection r = -1, which means 100% of the pulse is reflected as a Negative

pulse. If the fault resistance Z = Z0, the reflection r = 0; this fault occurs frequently on cables with lead

sheath and oil impregnated paper insulation. The cable may have been burnt to lower the fault resistance;

@ Z = Z0 no reflection exists. For an open circuit, Z is infinite and the reflection r = +1, which means

100% of the pulse is reflected a s a positive pulse.

Applications

Under field conditions, working with already buried lines, the reflection patterns are not always as well

defined and easily recognizable as shown in idealized graphs. Good runs of cable usually will show quite a

few reflections due to the changes of the uniform impedance along the cable run. These changes occurwhen splices, transformers, new cable is spliced in are introduced to the complete cable run. Interpretation

of these reflections comes with experience as well comparing distances to these changes to mapping

systems identifying these points. For fault location most TDR’s utilize a low energy pulse (TDR) to be

used to compare with the high-energy pulse (ARC Reflect) created by a thumper for distance measurements

to the cable fault.

Highly Resistive Faults

A TDR responds to changes of impedance and indicates by how much the impedance of an irregularity or

fault differs from the characteristic impedance of the cable. It should be noted that resistance and

impedance are different quantities. An insulation resistance tester or ohmmeter measures resistance only;

to measure impedance, in addition to the resistive elements, the reactive component needs to be determined

too. This reactive component is the capacitive and inductive elements of the cable design.

Resistive faults can be regarded as being in parallel to the characteristic cable impedance. The reflection

coefficient derives from the equation:

r = ___-_Z0___

2Z + Z0

If the cable has a characteristic impedance Z0 = 30Ω and it’s ohm reading is 100,000Ω. The reflection can

be calculated as: r=(-30)/(200,00+30)=0.00015 which is 0.015%. This reflection cannot be seen with a

TDR because other naturally occurring irregularities are considerably larger.

High resistance faults on cables with solid dielectrics cannot be seen with the low energy TDR alone.

Comparison of an Arc Reflective pulse to the TDR pulse is done to give a distance reference to the fault.

Arc Reflection TDR is used with a High Energy Capacitor Discharge system, which is needed in order to

break down the fault Resistive Fault.

5



Wet Sections of Cable

When a wet cable is checked with an insulation resistance tester, the resistance readings are generally very

high. This type of measurement often indicates a good cable. To a TDR, wet cable sections look very

different compared to the dry sections; wet sections are frequency dependent. This means that the higher

frequencies in the transmitted pulse of a TDR are affected much more than the lower frequencies of DC.

Fig. 6 shows a wet section of the cable that is marked.

Fig. 6 – Wet section of cable

Water and dissolved chemicals will change the dielectric properties of a cable and the velocity of

propagation. While the beginning of the wet section will show accurately, the end of it will not. To get

this measurement you would need to take a reading from the other end of the cable.

TDR Measurement Accuracy

Most TDR’s on the market today specifies a ±1% measuring accuracy. This is to be understood as 1% of

the range selected on the TDR. With 200 feet of cable, the accuracy will be ±2 ft. With 20,000 feet of

cable the measuring accuracy would be ±200 ft.

Unfortunately there are other sources that will contribute to the overall measurement error of these sets.

Even if the distance to the fault could be determined electrically with great accuracy, the task of physicallymeasuring that distance with the same accuracy becomes impossible. This is due to such variables as

hidden service loops, snake factor, jumping and skipping measuring wheel, etc.

Several techniques exist which can help to locate faults fairly precisely using a TDR even when the total

lengths of the cable and/or the propagation velocity are not known. These techniques are as follows.

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Three Stake Method

This method is an excellent technique to locating faults such as a severed or blown apart cable or a bolted

fault which a thumper can not be heard for pin pointing were the cable needs to be exposed for repair.

Using a TDR for measurement, the total length of cable or the propagation velocity do not have to be

known. With this method and others the location of the cable lie in the ground will need to be marked.

Procedure:

1. Guess the value of Vp /2, 250 ft.µs is a good average for most URD installation.

2. Adjust the TDR to this value

3. With the TDR, measure the distance to the fault from one end of the cable and drive a stake at

measured point.

4. From the opposite end use the TDR to measure the distance to the fault and drive a stake at

measured point.

5. Split the remaining distance, R between the two stakes by the ratio of the two measurements and

drive the third stake at this location. This will be were the cable fault lies in the ground.

See figure 7 below for example

R = 111 feet

From stake 1: ft ft ft

ft ft 79

4331063

1063111 =⎟⎟

⎠

⎞⎜⎜⎝

⎛ +

From stake 2: ft ft ft

ft ft 32

4331063

433111 =⎟⎟

⎠

⎞⎜⎜⎝

⎛ +

Fig. 7 Three Stake Method

Conclusion:The three stake method and a TDR is a easy way to locate and determine what type of fault there is in an

under ground cable system. This can help the user save time and money. For more information on cable

fault locating and using TDR technology please contact Hipotronics.

[email protected]

+1 845-279-3644

7

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basic tdr techniques paper 2010-5-13[1]

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