**7.1. Parametric studies of the dissipation factor**

Initial parametric study has pointed out three characteristic profiles of the dissipation factor, as shown in Figure 12. The occurrence of the resonant temperatures were dominant for unused but stored cables, since strong temperature dependencies were more related for cables with a long operation history.

According to Figure 6, a slight increase of the tan in the region of 30-40°C in Figure 12 a) and b) could be interpreted as an optimal temperature for particular polarization processes, and temperature dependent profiles as a dominancy of *con* tan . Differing to a temperature dependency that cannot be used as a diagnostic criterion (but must be considered), it is often assumed, that avoltage dependency is related to ionization processes and PD activity.

After approximately 6 weeks of artificial ageing the measurement procedure was repeated. The results are shown in Figure 13, respectively to Figure 12. The effects of the ageing on the absolute value of the dissipation factor are readily identifiable. Its increase is more intensive for cables with longer operation history, Figure 13 c). Moreover, the resonances are not

268 Dielectric Material

like tan

 *<sup>n</sup>* tan 

**7. Parametric studies** 

the aim of documenting the dependence of tan

**7.1. Parametric studies of the dissipation factor** 

According to Figure 6, a slight increase of the tan

cables with a long operation history.

processes and PD activity.

The initial condition of all cable samples was documented using PD analysis, tan(

measurements and return voltage measurements (RVM), (Mladenovic & Weindl, IEEE Electrical insulation Magazine, 2012). The resulting data are to be used as reference indicators for the later measurements. Anyway, the measurement of the PD is unique due to its local character, but should be correlated with measurements indicating the general cable condition

their complex and not negligible dependency on environmental and test parameters, (Weindl, Mladenovic, Scharrer, & Patsch, 2010) (Mladenovic & Weindl, Dependencies of the PD- and tan(δ)-Characteristics on the Temperature and Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Dependency of the Dissipation Factor on the Test-Voltage and the Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Comparison of the parametric Partial Discharges and Dissipation factor Characteristics of MV PILC Cables, 2012). For this reason series of parametric studies under controlled test conditions were made, with

condition, test voltage and cable temperature. In the following, some characteristic profiles of the dissipation factor and PD for different ageing groups will be presented and discussed.

The measurements, which enable the analyses of the diagnostic parameters dependency on the test and the cable conditions, were accomplished under defined and monitored environmental conditions, at the network frequency of 50 Hz, at selective test voltages in the range from 0.4 *Un* to 2.2 *Un* and within a temperature range from 10°C to over 90°C. All measured values

is the measured value on brand new (reference) cable at room temperature.

Initial parametric study has pointed out three characteristic profiles of the dissipation factor, as shown in Figure 12. The occurrence of the resonant temperatures were dominant for unused but stored cables, since strong temperature dependencies were more related for

a) and b) could be interpreted as an optimal temperature for particular polarization

temperature dependency that cannot be used as a diagnostic criterion (but must be considered), it is often assumed, that avoltage dependency is related to ionization

After approximately 6 weeks of artificial ageing the measurement procedure was repeated. The results are shown in Figure 13, respectively to Figure 12. The effects of the ageing on the absolute value of the dissipation factor are readily identifiable. Its increase is more intensive for cables with longer operation history, Figure 13 c). Moreover, the resonances are not

processes, and temperature dependent profiles as a dominancy of *con* tan

of the dissipation factor are presented in a normalized form of *<sup>n</sup>* tan tan

. However, a direct comparison of the measurement values is not possible due to

, PD and return voltage on the cable

in the region of 30-40°C in Figure 12

. Differing to a

, where

)

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 269

**Figure 12.** Selected initial profiles of tan showing a resonant temperature region (a) and the voltage and temperature dependency for the different cables

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 271

, and the corresponding portion of losses will be

present in the same way as in Figure 12 a) and b). Since these cables have no service history (brand new and 10 years stored cable, respectively), presence of the voids at the beginning of the experiment can be assumed. During the ageing process, mass allocation through the temperature variation proceeds and could lead to the grouping of the voids and fast failure or voids releasing in terminations, as it was shown in (Mladenovic & Weindl, Determination of the Environmental Conditions for the Accelerated Ageing of MV-PILC Cables, 2009). In further studies the PD activity,

Before PD profiles for specific cables are discussed, it is of the significant importance to affirm the evaluation and weighting of the single PD values out of the numerous impulses within one single measurement. For example, in Figure 14 two very different PD profiles from the same measurement cycle are presented. PD values are hereby evaluated from the impulses with highest repetition rate Figure 14 a) or from the average of the 50 maximal values Figure 14 b. In order to determine the profile which represents the "real" PD activity in a better way, each single measurement has to be

In Figure 15 single measurements for voltages *U*0 up to *U*<sup>0</sup> 2,2 with the step *U*<sup>0</sup> 0,2 and variable temperatures are presented. Concerning the region of high temperatures, where is the deviation between figures a) and b) (Figure 14) the most significant, and comparing them to the single measurements shown in Figure 15, it can be concluded that better correlation of the profiles is reached by a calculation as shown in Figure 14 b). Moreover, the analyses of other PD measurements were used to confirm this experimental

Finally, in the Figure 16 and Figure 17 PD-Temperature-Voltage profiles are shown respectively to the dissipation factor profiles in Figure 12 and Figure 13. It is obvious that a temperature region with amplified PD activity can be detected. Besides, only very intensive PD activity with a high density could cause losses within the material that would be recognizable in total dissipation losses. Due to the low absolute value of the initial dissipation factor this is visible by brand new cables, Figure 12 a) and

On the other hand, less intensive PDs in case Figure 16 b) and c) do not leave some obvious marks on the total dielectric losses in Figure 12 b) and c). Moreover, an amplified PD activity is present by brand new and stored-unused cable sample, what could confirm the

After six weeks of artificial ageing, there was a significant development of the PD activity, especially in the region of high temperatures. Anyway, it cannot be stated that areas

assumption of the void presence, as mentioned before.

simultaneously measured to the tan

**7.2. Parametric studies of PD** 

presented.

analyzed.

thesis.

Figure 16 a).

(b)

(c)

**Figure 13.** tan profiles from Figure 12 after six weeks of ageing

present in the same way as in Figure 12 a) and b). Since these cables have no service history (brand new and 10 years stored cable, respectively), presence of the voids at the beginning of the experiment can be assumed. During the ageing process, mass allocation through the temperature variation proceeds and could lead to the grouping of the voids and fast failure or voids releasing in terminations, as it was shown in (Mladenovic & Weindl, Determination of the Environmental Conditions for the Accelerated Ageing of MV-PILC Cables, 2009). In further studies the PD activity, simultaneously measured to the tan, and the corresponding portion of losses will be presented.

## **7.2. Parametric studies of PD**

270 Dielectric Material

**Figure 13.** tan

profiles from Figure 12 after six weeks of ageing

(a)

(b)

(c)

Before PD profiles for specific cables are discussed, it is of the significant importance to affirm the evaluation and weighting of the single PD values out of the numerous impulses within one single measurement. For example, in Figure 14 two very different PD profiles from the same measurement cycle are presented. PD values are hereby evaluated from the impulses with highest repetition rate Figure 14 a) or from the average of the 50 maximal values Figure 14 b. In order to determine the profile which represents the "real" PD activity in a better way, each single measurement has to be analyzed.

In Figure 15 single measurements for voltages *U*0 up to *U*<sup>0</sup> 2,2 with the step *U*<sup>0</sup> 0,2 and variable temperatures are presented. Concerning the region of high temperatures, where is the deviation between figures a) and b) (Figure 14) the most significant, and comparing them to the single measurements shown in Figure 15, it can be concluded that better correlation of the profiles is reached by a calculation as shown in Figure 14 b). Moreover, the analyses of other PD measurements were used to confirm this experimental thesis.

Finally, in the Figure 16 and Figure 17 PD-Temperature-Voltage profiles are shown respectively to the dissipation factor profiles in Figure 12 and Figure 13. It is obvious that a temperature region with amplified PD activity can be detected. Besides, only very intensive PD activity with a high density could cause losses within the material that would be recognizable in total dissipation losses. Due to the low absolute value of the initial dissipation factor this is visible by brand new cables, Figure 12 a) and Figure 16 a).

On the other hand, less intensive PDs in case Figure 16 b) and c) do not leave some obvious marks on the total dielectric losses in Figure 12 b) and c). Moreover, an amplified PD activity is present by brand new and stored-unused cable sample, what could confirm the assumption of the void presence, as mentioned before.

After six weeks of artificial ageing, there was a significant development of the PD activity, especially in the region of high temperatures. Anyway, it cannot be stated that areas

#### 272 Dielectric Material

with maximal PD activity correspond to those of highest dissipation losses. For example, very strong PD activity is present in the area of higher temperatures and voltages in Figure 17 b). On the other hand, the dissipation losses of this cable show a dominant voltage dependency and almost no temperature dependency except on high voltages, whereby the maximal value lay in the region of low temperatures - differing therefore to PDs.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 273

**Figure 15.** Background of PD profiles in Figure 14 - single PD measurements for variable test-voltages

and temperatures

**Figure 14.** Two different PD profiles calculated from the same measurement cycle a) evaluated from the values with the highest repetition rate and b) evaluated from the average of the 50 maximal PD impulses

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 273

272 Dielectric Material

PDs.

impulses

with maximal PD activity correspond to those of highest dissipation losses. For example, very strong PD activity is present in the area of higher temperatures and voltages in Figure 17 b). On the other hand, the dissipation losses of this cable show a dominant voltage dependency and almost no temperature dependency except on high voltages, whereby the maximal value lay in the region of low temperatures - differing therefore to

**Figure 14.** Two different PD profiles calculated from the same measurement cycle a) evaluated from the

(b)

(a)

values with the highest repetition rate and b) evaluated from the average of the 50 maximal PD

**Figure 15.** Background of PD profiles in Figure 14 - single PD measurements for variable test-voltages and temperatures

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 275

(a)

(b)

**Figure 17.** PD profiles simultaneously measured to the tan

(c)

shown in Figure 13

(b)

**Figure 16.** PD profiles simultaneously measured to the tanacquisition shown in Figure 12

274 Dielectric Material

**Figure 16.** PD profiles simultaneously measured to the tan

(c)

(a)

(b)

acquisition shown in Figure 12

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 275

**Figure 17.** PD profiles simultaneously measured to the tanshown in Figure 13
