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PD measurements can provide a significant insight into the presence of harmful defects that can damage the cable insulation.
Usually electrical installations undergo a Time Based Maintenance (TBM) program at fixed time intervals. However, for the most critical equipments a Condition Based Maintenance (CBM) is much more advisable. Applying CBM means to keep the electrical equipment state under control. This is done to detect potential sources of problems at a very early stage and to take the relevant countermeasures before irreversible and harmful events happen. By means of CBM the failure rate is kept constant, at a level chosen by the asset manager and, therefore, long forced stops can be avoided, thus limiting costs due to loss of revenues and repair after failure.
PD measurements on MV cable systems can be carried out both on-line, when the equipment under test is supplied by the electric network, and off-line, with the electrical system supplied by an external voltage source.However, off-line tests can be carried out only periodically, when the circuit can be switched off and hiring an external source, thus at high cost. Moreover, stress profiles in offline tests may be noticeably different from those experienced in service.
After years of advanced researches, laboratory and on-field tests, Techimp has developed an on-line powerful and effective technology which supports the asset manager decisions regarding transmission and distribution cable maintenance and reliability.
Trending of PD phenomena during cable system life through on-line PD measurements (in permanent or non-permanent way) can be a very effective method to keep the degradation processes under observation and control, particularly if also quantities other than PD are monitored at the same time (e.g. temperature, current, dissipation factor).In such a way, correlation between possible PD inception with load and thermal cycling can be derived and other degradation modes, as the presence of water trees, are surveyed.
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