Measurement methods overview

Time Domain Reflectometer (TDR) is a cable device that uses the principle of a radar. The short probing pulse is supplied to the cable line. The signal propagates along the cable and reflected partially or completely at the cable impedance heterogeneity and finally returns back to the input of the device. After processing a received signal device displays the result in the metric system of measures on the screen.

1.  Time Domain Reflectometry (TDR) Method

The instrument uses a Time Domain Reflectometry method (TDR), which is based on the phenomenon of a partial reflection of electromagnetic waves by the impedance irregularities in the line. When applying the TDR method a rectangular probe pulse generated by the pulse generator is sent into the line. Probe pulse propagating through the cable line, completely or partly reflected from the fault area in the line (impedance irregularities) and returns to the receiver input. Reflection waveform is observed on the screen and allows to determine the type of the fault (see Table 2) and the distance to it. The reflected pulses are returned to the device after a certain time from the moment of sending the probe pulse.
Knowing the speed of electromagnetic wave propagation along the line, and the time delay of the reflected signal (measured by the device), we can calculate the distance to the impedance irregularity.

 f1

where,

L – Distance to the impedance irregularity, m.

v – Propagation speed of electromagnetic wave in the line, m/µs;

PF – Propagation Factor, c/v

VoP – Velocity of Propagation , (v/c)*100%

td – Measured time delay of the reflected signal, µs;

с – Speed of light is equal to 299,8 m/µs;

 

Impedance irregularities are the result of violations manufacturing technology of cables as well as a consequence of mechanical and electrical hazards during the construction and operation of the cable lines.  Any cable device (couple, split, joint, Pupin coil, etc.)  cause irregularity of the line impedance.

The impedance irregularities may be caused by any cable devices (couple, joint, Pupin coil, etc.) or faults (open, short, partial open, partial shot, wetting core of the cable, leaks to the ground, split pairs, etc.). TDR method allows to fix multiple irregularities both lumped and lengthy, depending on the ratio of their length and the minimal wavelength of the spectrum of the probe pulse.

The instrument generate a probe pulse with positive polarity amplitude is not less than 10V (TDR RI-407 in U1 mode), or not less than 45V (TDR RI-407 in U2 mode) . The probe pulse width is automatically adjusted according to the selected sub-range (see Table 1). Furthermore, pulse width can be set manually by the user.

The instrument automatically calculates the distance, according to the velocity factor and measurement cursors positions on the screen. The distance measurement error is determined by the sampling step of the instrument and by the velocity factor setting error.

Sampling step for each sub-range is set by default in such a way that the viewing window got whole sub-range used by (see. Table 1 for TDR RI-407). Sampling step can be adjusted (reduced or increased) manually by the operator to minimize the instrumental error.
The velocity factor value is depends of the type of cable.

f2

where,

PF – Propagation Factor;

v – Propagation speed of electromagnetic wave in the line, m/µs;

с – Speed of light is equal to 299,8 m/µs;

ε – Dielectric constant of the cable insulation.

Propagation speed can be determined experimentally, knowing in advance the distance (L) to any irregularity (e.g., cable length or distance before the coupling). In this case, the inverse problem is solved that way:

f3

f4

Additional errors are due to the distortion of the reflected signal in the lines with a frequency-dependent losses. The measurement error affects the nature of irregularity, its value, the existence of several irregularities in the line.

 

Table 1 Sub-bands-measurement range TDR RI-407

Sub-range, m Default pulse width, ns Sampling step, mPF=1.5 (VOP=66%) Minimal sampling step, mPF=1.5 (VOP=66%)
0 – 62,5 10 0,125 0,125
0 – 125 10 0,250 0,125
0 – 250 20 0,500 0,125
0 – 500 50 1,000 0,125
0 – 1000 100 2,000 0,250
0 – 2000 200 4,000 0,500
0 – 4000 500 8,000 1,000
0 – 8000 1000 16,000 2,000
0 – 16000 2000 32,000 4,000
0 – 32000 5000 64,000 8,000
0 – 64000 10000 128,000 16,000
0 – 128000 20000 256,000 32,000

 

Table 2 Waveforms of the typical irregularities.

Waveform Description
 1 The first cursor points to the probe pulse.The second cursor points to the reflection from irregularity with  high impedance, which corresponds to the cable break (COMPLIT OPEN).
 2 The first cursor points to the probe pulse.The second cursor points to the reflection from irregularity with  low impedance (reflection with inversed signal polarity), which corresponds to a short circuit in the cable (DEAD SHORT).
 3 The first cursor points to the probe pulse.The second cursor points to the reflection from irregularity with  increased impedance, followed by a complete cable break. (PARTIAL OPEN)
 4 The first cursor points to the probe pulse. The second cursor points to the reflection from irregularity with  decreased impedance, followed by a complete cable break (PARTIAL SHORT).
 5 The first cursor points to the probe pulse. The current waveform shows three joints on a cable. A joint, marked by the second cursor is defective, it can be clearly seen on the level of reflection.
 6 The presence of a faulty amplifier in the line results an increased reflection from the amplifier. The signal must terminate on the amp, but it’s possible an additional reflection (phantom) of the amplifier.
 7 Couplers in the cable can cause measurement errors due to multiple reflections. On the waveform cursor marks the coupler. Two differently directed reflected signals indicate the two segments of the coupler.
 8 Additional resistance or weld rise to an S-shaped reflection on the trace. Reflection from  increased impedance followed by reflection from  decreased impedance.
 9 Well matched cable including cable terminator fully absorbs signal reflections. This waveform guarantees the normal cable terminator choice, which does not cause reflection.
 9 Wet cable is recognized as an area with the random reflection. The beginning of this area shows a second cursor corresponds to the beginning of the wet region.
 10 Increasing humidity in the cables leads to the appearance of the noises in the waveform.

 

Note to Table 2: The amplitudes of the pulses are given in the appropriate proportions at the same amplification.

 

2.  Arc Reflection Method (ARM)

Localization of the cable faults with a high transient resistance (R>10 kOhm) is usually difficult when using low-voltage TDR method. One of the ways for localization such defects in power cables is an Arc Reflection Method (ARM).

Clip_2

Implementation of ARM method is carried out with the additional equipment: high-voltage pulse generator (HVPG) and the special connecting device (as an external unit or internal HVPG unit).

The essence of the ARM method is in creating conditions (using HPVG)  for the occurrence electric arc (breakdown)  for a short time (few milliseconds) in the point of the fault. Synchronously with the burning arc (sync signal obtained from HPVG) reflectometer performs sensing. The TDR’s probe pulse is reflected from a low resistance of the arc with inverted polarity (like shorted circuit).

m500p50a1g0U2_ARC

For easy identification of the fault location, waveform without breakdown (high transient resistance at the point of the defect) and waveform during breakdown (low transient resistance at the point of the defect) are compared.

3.  Oscillatory Discharge Methods (ODM)

Localization of the cable faults caused self-healing insulation breakdown is usually difficult when using low-voltage TDR method. One of the ways for localization such defects in power cables is an Oscillatory Discharge Methods (ODM): Impulse Current Method (ICM) and Decay travelling wave method (DECAY).

Clip_3

The methods of oscillatory discharge (ICM, DECAY) are based on the measuring period of oscillatory processes occurring in the breakdown of the charged cable. Implementation of the methods is carried out using the optional equipment: the high-voltage pulse generator (HVPG) and a special connecting device (as an external block or internal HVPG unit).
The essence of the method is: HVPG raises voltage in the cable until the breakdown occurred. The defect causes a breakdown of the insulation at the site of the fault, causing a spark, which has low resistance and the oscillatory discharge in the cable occurs. Knowing the speed of electromagnetic wave propagation (v) in the line and the period of oscillation process (Top), we can calculate the distance to the breakdown (L):

f5

To achieve the highest accuracy only the first oscillation period is selected.

m1000g160_WAV