Deterioration of the underground power cables insulation has been established to be caused by electrical, thermal and environmental stresses. With the degradation of dielectric strength of the insulation, the underground cables will not be able to function optimally or as planned. As a supplement to the existing dissipation factor and other conventional cable diagnostic measurements, the Decay Voltage (DV) and the Return Voltage (RV) measurement technologies are currently being proposed for nondestructive diagnosis of cable insulation. The main purpose of this paper is to present results from laboratory measurements of DV, RV and Polarisation and Depolarisation Current (PDC) on a number of samples of oil-impregnated paper insulated cables. Comparison of these results has also been made with the conventional dissipation factor measurement data for the same cables

Purkait, P.

Saha, T. K.

Yew, J. H.

University of Queensland eSpace

Experience with dielectric response measurements on oil-paper insulated cables 
T.K. Saha, J.H. Yew and P. Purkait  
University of Queensland, Brisbane, Queensland, Australia 
Abstract: Deterioration of the underground power 
cables insulation has been established to be caused by 
electrical, thermal and environmental stresses. With 
the degradation of dielectric strength of the insulation, 
the underground cables will not be able to function 
optimally or as planned. As a supplement to the 
existing dissipation factor and other conventional 
cable diagnostic measurements, the Decay Voltage 
(DV) and the Return Voltage (RV) measurement 
technologies are currently being proposed for non-
destructive diagnosis of cable insulation. The main 
purpose of this paper is to present results from 
laboratory measurements of DV, RV and Polarisation 
and Depolarisation Current (PDC) on a number of 
samples of oil-impregnated paper insulated cables. 
Comparison of these results has also been made with 
the conventional dissipation factor measurement data 
for the same cables. 
1. Introduction 
The widespread use of underground cables in the 
high-voltage (HV) power industry today stems from 
the many advantages of using underground cables in 
preference to using overhead lines. Nevertheless a 
common existing problem encountered by the industry 
is determining the condition of the insulation of the 
underground cable. Over the period of time, it is 
inevitable for insulation materials to undergo chemical 
and physical deterioration, causing the performance of 
the cables to suffer. Such deterioration will weaken 
the dielectric strength of the insulation, resulting in a 
decline in the condition of the cables and 
subsequently, decreasing the life span and efficiency 
of the underground power cable.   
 Several traditional and the newer diagnostic testing 
methods, such as dielectric loss, absorption and 
dispersion coefficient measurements and Partial 
Discharge (PD) Localization by Time-Domain 
Reflectometry [1] have been used over the years for 
cable testing purposes. However, the relation between 
these measured quantities and the fundamental 
insulation degradation processes have not been 
explored adequately. This has many times lead to 
incorrect or contradictory interpretation of the results 
[2]. 
 To identify the two fundamental characteristics of 
the cable insulation material, namely the conduction 
and the polarisation, and hence to assess its condition, 
non-destructive dielectric diagnostic techniques are 
currently being investigated [3-7]. These include the 
Decay Voltage (DV) measurements and the Return 
Voltage Measurements (RVM). 
 The present paper reports the investigation results 
of insulation diagnostics on several oil-paper insulated 
cables by traditional techniques like the dissipation 
factor (tanδ) measurement, newer techniques like the 
DV and RV measurements and also a relatively new 
dielectric diagnostic technique, namely the 
Polarisation and Depolarisation Current (PDC) 
measurement [8-10].  
2. Dielectric Diagnosis Techniques [2, 4-10] 
The different dielectric diagnostic measurements 
including DV, RV and PDC can be performed on the 
three-core cable using the basic circuitry as shown in 
Figure 1. 
Figure 1: Circuit for dielectric diagnosis  
 The Decay Voltage (DV) is measured between the 
two electrodes after charging the insulation for 
sufficiently long time (~1000sec) and the removing 
the voltage source as described in Figure 2(a). The 
return voltage (RV) is measured between the two 
electrodes after a period of short circuit following a 
longer period of charging. In the PDC technique, the 
charging current due to a DC step voltage and the 
subsequent discharging current after replacing of the 
voltage source by a short-circuit are measured for long 
periods of time. The process of RV and PDC 
measurements are combined in Figure 2(b).  
(a)
I
1
(b) 
Figure 2: DV, RV and PDC measurements,  (a) DV curve (UD) and 
its initial slope (SD), (b) RV curve (UR) and its initial slope (SR) and 
PDC curves (ip and id)
It has been shown [4, 6] that the steepness of the 
decay voltage curve (SD) is proportional to the 
intensity of conduction, while the steepness of the 
return voltage curve (SR) is proportional to the 
intensity of polarization process. The specific 
conductivity, γ [A/V] and the polarization 
conductivity, β [A/V] can thus be computed from SD
and SR respectively using (1) and (2): 
OD ES εγ /=              (1) 
OR ES εβ /=           (2) 
where 
SD is the initial slope for decay voltage curve 
SR is the initial slope for return voltage curve 
E is the applied DC electric field [V/m], and 
εO is the permittivity of vacuum 
 The RV Spectrum is obtained from multiple cycles 
of the RVM with increasing charging times. The peak 
of the maximum RV among all the cycles and the 
corresponding time, called the central time constant 
(CTC) are parameters used for assessment of the 
insulation condition [6].   
 A newer technique used to monitor the condition 
of the insulation is the Polarisation-Depolarisation 
Current (PDC) measurement.  From the values of the 
measured polarisation and depolarisation currents (ip
and id), the oil conductivity and the paper conductivity 
can be measured and studied to evaluate the oil 
condition and the paper condition in the insulation [8-
10]. Equation (3) is used to calculate the conductivity: 
( )dp
r
O
O ii
C
U
−=
ε
ε
σ                             (3) 
where 
Uo is the charging voltage, C is the measured 
capacitance of the cable and İr is the combined 
relative permittivity of oil and paper insulation. 
The oil conductivity is calculated from the initial 
values of the polarisation and depolarisation currents, 
whereas the paper conductivity is calculated from the 
final values of the polarisation and depolarisation 
currents.
3. Experimental Techniques 
Figure 3 shows five different oil-paper insulated cable 
samples collected from a university distribution 
system with no prior history known to the authors. 
The lengths of the cables are given in Table 1. 
Figure 3: Oil-paper Insulated Cable Samples 
Dielectric response measurement equipment, 
which was developed at the School of Information 
Technology and Electrical Engineering, at the 
University of Queensland [9-10], was used for all the 
dielectric response measurements. For Dissipation 
Factor measurement, it was done with Capacitance 
and Dissipation/Power Factor Test Set Type 2816, 
Tettex Instruments. 
4. Results and analysis 
Dissipation Factor (tan δ) measurement 
The dissipation factor at 50 Hz was measured at 500V 
and the results are shown in Table 1. 
Table 1: Dissipation Factor 
 Cable   Length [m]   tan δ
 A     1.6     0.0116 
 B     1.5     0.0569 
 C     0.8     0.0047 
 D     0.9     0.0035 
 E     1.0     0.0033 
Cable B having the highest dissipation factor 
indicates that the condition of the insulation in cable B 
is in a degraded state. Cables C, D and E have smaller 
dissipation factors which imply that the average 
insulation conditions in these three cables are 
relatively good. The dissipation factor for cable A is 
between cable B and Cables C, D and E. Thus, the 
insulation condition of cable A is moderate.
Decay Voltage measurement 
Figure 4 shows the decay voltage curves obtained 
from the experiment for the five cable samples.  
A
B
C
D
E
2 Saha, T.K., Yew, J.H. & Purkait, P. 
Figure 4: Decay Voltage measurement results 
Based on the decay voltage curves, cable B 
produced the steepest gradient while cables C, D and 
E produced the lowest initial gradient. Since there is a 
strong correlation between the initial tangent and the 
amount of moisture in each individual cable, the 
insulation condition of cable B is in a much advanced 
stage of insulation degradation as compared to the 
relatively good cables C, D and E. Along with that, 
the condition of the insulation for cable A is 
intermediary of cable B and cables C, D and E. 
 Table 2 contains the initial slopes and the 
corresponding specific conductivities for all the five 
cable samples.  
Table 2: Specific Conductivities from Decay Voltage slope
 Cable   SD [V/s]    γ [A/V] 
 A    1.97     1.75E-13 
 B    27.2     2.41E-12 
 C    0.07     5.71E-15 
 D    0.09     7.99E-15 
 E    0.11     9.51E-15 
As seen in Table 2, the cable B is having the 
highest value of specific conductivity – indicating its 
degraded condition of insulation. Whereas cables C, D 
and E are in good condition with low values of 
specific conductivities. Cable A lies in-between. 
Single Cycle Return Voltage measurement
The return voltage curves obtained from the 
measurement with 1000 sec charging and 1 sec 
discharging are illustrated in Figure 5 while Table 3 
shows the initial slopes and the polarisation 
conductivities for the five cable samples. 
Table 3: Data obtained from return voltage measurement 
 Cable       SR [V/s]    β [A/V] 
 A     4.07     36.2E-14 
 B     2.14     18.9E-14 
 C     0.32     2.87E-14 
 D     0.22     1.91E-14 
 E     0.25     2.21E-14 
Figure 5: Return Voltage measurement results 
Cable A and B have quite high values of the 
polarization conductivity – which is indicative of their 
degraded condition of insulation. Some anomaly of 
the values of SR and β were found between cables A 
and B. As can be seen from Figure 5, though the 
initial slope of cable B is lower than cable A, cable B 
has a very low value of CTC – which denotes worst 
condition of its insulation than all the others. On the 
contrary, cables C, D and E have low polarization 
conductivity values. Hence, it can be deduced that all 
these three cables C, D and E are in better conditions 
as compared to cables A and B. 
Return Voltage spectrum 
Figure 6 shows the RV spectrum for the cable samples 
and Table 4 shows the measured parameters obtained 
from the measurement. Cable B is not available for 
this measurement as it was suspected to have 
developed some permanent damage during the 
previous test. 
Figure 6: Polarisation Spectrum 
Table 4: Data obtained from PDC measurement 
 Cable    Max RV (V)   CTC (s)  
 A     16.91     39.6   
 B     N.A.     N.A.   
 C     4.93     537.83   
 D     3.23     426.66   
 E     6.04     690.8   
3Experience with dielectric response measurements on oil-paper insulated cables
From the data, Cable A can be seen to have the 
smallest CTC while all three cables C, D and E have 
large CTC. This again confirms that the condition of 
insulation for cable A is worse than cables C, D and E. 
Polarisation-Depolarisation current measurements 
Figures 7 and 8 show the polarisation and 
depolarisation current plots respectively for all the 
cable samples. Using (3), the calculated values of oil 
and paper conductivities (σO and σP) for the cable 
samples are shown in Table 5. Again, Cable B is not 
available for this measurement.  
Figure 7: Polarisation Currents
Figure 8: Depolarisation Currents 
Table 4: Data obtained from PDC measurement 
Cable    σO (S/m)    σP (S/m) 
A     1.11E-14    9.84E-15 
B     N.A.     N.A. 
C     3.00E-15    1.05E-15 
D     3.21E-15    1.13E-15 
E     3.02E-15    6.69E-16 
The higher values of the polarisation and 
depolarisation currents for cable A as compared to the 
other three cables C, D and E indicate that cable A 
insulation is in a more degraded condition than the 
other three. This is supported by the fact that cable A 
has higher oil and paper conductivities as compared to 
the other cables, as shown in Table 4. A higher 
conductivity obviously implies a degraded condition 
of the insulation. From this measurement, it is thus 
once again confirmed that the insulation of cables C, 
D and E are in better condition among all the cable 
samples. 
From the above measurements, we can thus 
observe a good agreement between the results of all 
the different dielectric diagnostic techniques as well as 
the traditional dissipation factor measurement results.
5. Conclusions
Five different oil-paper insulated cable samples have 
been tested with different dielectric techniques like the 
Decay Voltage, Return Voltage and PDC 
measurements. The results obtained show that the 
non-destructive diagnostic techniques proved to be 
reliable, effective and efficient in determining the 
insulation condition of the oil-paper cable samples. 
Different parameters like specific conductivity, 
polarisation conductivity, CTC, oil and paper 
conductivities have been calculated from the different 
measurements and have been used for assessing the 
condition of the cable insulation. The results obtained 
from these dielectric tests are in good agreement with 
the values of the dissipation factors for all the cables.  
6. References
[1] Z. Du, P. K. Willett and M. S. Mashikian, “Performance 
Limits of PD Location Based on Time-Domain 
Reflectometry”, IEEE Trans. Dielectrics and Electrical 
Insulation, vol. 4, no. 2, pp. 182-188, 1997. 
[2] E. Nemeth, “Measuring the Voltage Response, A Diagnostic 
Test Method of Insulations”,  7th International Symposium on 
High Voltage Engineering, Dresden, Paper No. 72.06, pp.63-
66, 1991. 
[3] E. Nemeth, “Practical Experiences of Diagnostic Testing of 
Power Cable Lines by the Voltage Response Method”, 40. 
IWK, TU Ilmenau, Band 4, pp. 699-708, 1995. 
[4] E. Nemeth, “Some Newest Methods of Diagnostic Testing of 
Impregnated Paper Insulated Cables”, 10th International 
Symposium on High Voltage Engineering, Canada, 1997. 
[5] M. G. M. Megens, A. Rama and R. Ross, “Return Voltage 
Measurements on Paper insulation of Windings and Cables”, 
IEEE International Conference on Conduction and 
Breakdown in Solid Dielectrics, Sweden, pp. 98-101, 1998 
[6] E. Németh, “Measuring Voltage Response: A Non-destructive 
Diagnostic Test Method of HV Insulation”, IEE Proc.–Sci. 
Meas. Technology., vol. 146, no. 5, pp. 249-252, September 
1999. 
[7] R. Neimanis, T. K. Saha and R. Eriksson, “Determination of 
Moisture Content in Mass Impregnated Cable Insulation Using 
Low Frequency Dielectric Spectroscopy”, IEEE Power 
Engineering Society Summer Meeting, vol. 1, pp. 463 -468, 
2000. 
[8] J. J. Alff, V. Der Houhanessian, W. S. Zaengl, and A. J. 
Kachler, “A Novel, Compact Instrument for the Measurement 
and Evaluation of Relaxation Currents Conceived for On-site 
Diagnosis of Electric Power Apparatus” IEEE International 
Symposium on Electrical Insulation, pp. 161 -167, 2000. 
[9] T.K. Saha, P. Purkait and Z.T. Yao, “Condition Monitoring of 
Transformer Insulation by Polarization and Depolarisation 
Current Measurements”, Australasian Universities Power 
Engineering Conference, Melbourne, Australia, 2002. 
[10] Z. T. Yao and T. K. Saha, “Separation of Ageing and Moisture 
Impacts on Transformer Insulation degradation by Polarisation 
Measurements”, CIGRE Paris session, paper no. 15-304, 
2002. 
4 Saha, T.K., Yew, J.H. & Purkait, P. 


Experience With Dielectric Response Measurements On Oil-Paper Insulated Cables

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Experience With Dielectric Response Measurements On Oil-Paper Insulated Cables

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