With the support of the Dutch government and two large Dutch grid owners, Continuon and Eneco Netbeheer (together responsible for about 65,000 km [40,3891 miles] of medium-voltage cable), KEMA and the Technical University of Eindhoven have developed a method to continuously monitor cables while they remain on-line.

Aging causes deterioration that, over time, can lead to failure. Aging can lead to the development of tiny electrical arcs that can be detected. Cable failure is actually preceded by such partial discharges (PDs). The timely identification of these PDs and repair of the cable section where PD is detected can prevent many failures. To find sections of the cable where deterioration is occurring, off-line diagnostic techniques have been in use for a decade.

In 2000, KEMA and the Technical University of Eindhoven began the investigation of the Partial Discharge On-Line (PDOL) project for medium-voltage power cables. Four years later, the working principle has been proven. Now, the project team is working on additional field tests and is constructing the first production series of devices. In order to measure PD, sensitive detection coils are installed at the ends of the power cable being tested in two substations, or Ring Main Units. The measured signals are captured in a small portable computer and reduced data is sent via a (cellular) telephone connection to a central server that calculates whether there are problems in the cable and, most importantly, where they are located.

Corrective actions are only possible if the locations of these weak spots are known. Furthermore, the interpretation of measurement results greatly benefits from knowledge about the PD origin, and thereby the type of the PD-producing cable or accessory. This is addressed by applying two detection coils, one at each cable end. Because short-duration signals propagate toward both ends of the cable, a measurement of the difference in arrival times can indicate where the faulty cable section is located. The necessary accurate-time synchronization between the two measurement setups is established by means of a patented pulse-injection system. The pulses are injected in the cable and serve as an accurate time reference for the measured signals. The advantages of on-line monitoring and localization of weak spots in cables are numerous. PDs do not always occur continuously, so off-line measurements can miss certain weak spots. On-line measurements can be performed continuously, capturing every occurrence of PD activity. Furthermore, information on weak spots and their location in a cable circuit can now be obtained without switching actions. Such diagnostic tests can be carried out whenever the grid owner wants. The measurement system only has to be installed on the two above-ground termination points in the connected substations in order to perform its task. And for instance, for known critical cable circuits, a system can be installed permanently.

These factors all reduce maintenance expenses. The grid manager can take timely action and make responsible investment decisions. And above all, it improves the reliability of the medium-voltage network.

PDOL has been tested on a test cable circuit that grid managers installed on the grounds of the KEMA facility. A Dutch manufacturer has been selected to produce the first production series of the measuring instruments before the end of the year. In 2006, the large-scale introduction for grid managers should be able to take place.

For more information, contact mark.mplewijk@kema.com.