The type and location of defects affect the interpretation of PD data. It is well known that defects in cable systems can result in premature failure, so it is important to be able to locate the defects and to determine the type of defect. Defects may be inadvertently introduced into cables or accessories during manufacture, storage, handling, transportation, and installation, or they may develop as the cable system ages in service while exposed to electrical, mechanical, thermal, and environmental stresses. Some defects do not produce partial discharges.
Online PD testing is the process of testing for PD while the cable is in service. This type of testing is non-invasive and does not require the cable to be de-energized.
Online PD testing is performed using sensors that are placed on the cable or near the cable terminations. High-Frequency Current Transformer (HFCT), Transient Earth Voltage Sensor (TEV), High Voltage Coupling Capacitor or Acoustic Sensor. Some of these sensors can be installed while the system is online while others require an outage of the cable system to install them.