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Stay Online With Partial Discharge Testing
How to maintain continuous power supply while measuring for weak spots.
the customers. For example, a KEMA study at a major U.S. investor-owned utility predicted that the number of cable failures on its system dramatically would increase in the coming years unless certain preventative measures are taken. Currently, this utility experiences about 600 cable failures each year on an installed base of approximately 25,000 circuit miles of several different types and vintages of cables. Based on a statistical analysis of the data, this number could increase by a factor of 10 over the next 30 years. An average customer served by this utility currently experiences about 167 interruption minutes per year, with 13 interruption minutes being caused by cable outages. The number of interruption minutes is likely to increase to more than 280 minutes if only a reactive cable repair policy is employed, thereby significantly reducing the reliability of electric service to its customer base.
The PD-OL methodology has been tested and developed by KEMA in Europe and is entering the pilot stage in the United States. The system locates the origins of PDs by using two inductive sensors, each at one cable end. A patented solution is used for the time synchronization of the data intake at both cable ends and for the online calibration. One computer at a remote location collects data from various PD-OL measuring systems for final interpretation and presentation, which can be made visible on a secured Web site for the network owner.
PD measurements can help to diagnose medium-voltage cables. The offline test systems applied today can measure and locate PDs by placing one sensor at one cable end and using PD pulse reflection. This implies that the cable has to be disconnected from the grid and energized with a separate power supply to generate the PDs. The primary reason PDs currently are not measured and located online is that under online conditions, PD pulses barely will reflect at the cable ends, making localization with one sensor practically impossible. The reflections become too small to detect because of the non-infinite impedance of the equipment connected to the cable under test at the transformer house or substation. Furthermore, measuring with one sensor at one cable end, it is impossible to distinguish pulses from the cable under test and pulses originating from other adjacent equipment.
For online PD detection and localization, the only effective solution is placing sensors at both cable ends illustrated in Fig. 1. While this solution may look simple, achieving accurate time synchronization of the digitizers at both cable ends is problematic. In the case of a required PD localization accuracy of about 1 percent of the cable length, the related time synchronization of the digitizers should be in the order of 100 ns for a 2 km long. For this synchronization, KEMA has a known and patented solution with GPS, which is used for offline PD measurements on long (up to 10 km) and branched cable networks. However, this is an expensive and impractical solution for long-time monitoring. A possible alternative, as suggested by many, is the use of atom clocks, but these