THE RELIABILITY OF A DISTRIBUTION NETWORK, coupled with the need to improve the management of these networks, is a problem utilities around the world face. Improved system reliability is beneficial to the relationship with customers and it reduces operating costs. Furthermore, there is a growing concern about the increasing age of the distribution networks and considerable pressure to realize savings through maximum-efficiency network management, a strategy that means asset replacement only when strictly necessary.

This strategy questions which components of a distribution network must be replaced and when. The cost to replace underground cable circuits with the need for excavation, in particular, is extremely expensive; hence, network managers must have indisputable evidence to justify necessary replacement. The interruptions in supply that customers experience are often related to component failures originating in the medium-voltage (MV) network. Frequently, the aging of cables and cable accessories — resulting from incorrect installation, moisture ingress, corrosion and thermal overloading — causes the interruptions. Aging causes weak spots that often take time to materialize, and diagnostic techniques have been used for nearly 10 years to measure and locate weak spots in the MV cable network.

In Holland, KEMA and the Technical University of Eindhoven — with the support of the Dutch government and two large network owners, Continuon and Eneco Netbeheer, who together are responsible for some 65,000 km (40,400 miles) of MV cable — have developed an on-line measuring system called Partial-Discharge On-line Cable Test with Localization (PD-OL).

OFF-LINE MEASUREMENT

The off-line measuring technique for finding weak spots in cables has been proven, having been used throughout the world during the past decade, but it invariably requires network switching to maintain customer supplies during the test period. The equipment, which often has to be mounted in a special-purpose vehicle, determines the presence of weak spots and measures the magnitude of the partial discharges that often precede cable failure.

This labor-intensive, time-consuming technique is impractical when a wide-scale network survey is required. As partial discharges are not necessarily continuous, a disadvantage of this technique is that the measurements are only made periodically; hence, there is no guarantee that all weak spots have been identified. Figure 1 illustrates this problem, showing the variations in partial discharge measured on an on-line cable circuit. The partial discharges are not present continuously.

ON-LINE MEASUREMENT

In 2000, KEMA and the Technical University of Eindhoven began an initiative that led to the permanent monitoring of cables. This partial-discharge on-line project for MV cables, primarily rated in the 6-kV to 50-kV voltage range, started in 2001, in cooperation with Continuon and Eneco Netbeheer with support from the Dutch government. During the past four years, the working principle has been proven. Now the project team is working on additional field tests and construction of the first production series.

DETECTION AND NOISE

In order to measure partial discharges, sensitive detection coils are installed at the end of the cable under test in two substations or on Ring Main Units (RMUs). The measured signals are captured in a small PC and reduced data are sent via a (cellular) telephone connection and Internet to a central server that calculates any problems in the cable and, most importantly, where they are located.

The complexity of this method is found, first and foremost, in the measurement and interpretation of the partial discharges. A discharge from a cable or cable splice (joint) is often no larger than a few millivolts. This is almost negligible in comparison with the energization voltage (10 kV) of the cable. It is extremely challenging to detect the desired signal among the noise from sources such as the switching of thyristors and other disruptions. Hence, the design used has been directed toward the signal-processing methods.

LOCATION

Detection of the partial discharges is only the beginning as the discharges can come from many sources, for example, from connected transformers or from other connected cables. Correct interpretation of the measurement results and corrective actions such as the repair of weak spots are only possible if the locations of these weak spots are precisely known — a problem solved by using two detection coils, one at each cable end. Because the short-duration signals propagate toward both ends of the cable, a measurement of the difference in arrival times can indicate where the weak area is located. Since the signals propagate through the cable at approximately half the speed of light, it is necessary to synchronize the two measurement systems with a difference of less than 100 nsec.

To achieve this, a special synchronization technique was developed that has since been patented. From both ends, high-frequency pulses are artificially injected into the cable. These clock pulses serve as a time reference for the measured partial-discharge signals. This makes the synchronization of the two measurement systems at both cable ends possible, through which the time difference and the location can be calculated. Processing of the measurements, including display, database storage and interpretation by specialists, occurs at a remote location. Although interpretation of the signals is a specialization, with the assistance of “specialists” and knowledge rules, network managers can take action in a timely manner.

It should be appreciated that a specialist or staff with the appropriate training can correctly diagnose the partial-discharge signals. For example, a small partial discharge in a resin-insulated splice requires immediate action, while the same partial discharge in an oil-insulated splice is of no concern.

TEMPORARY OR PERMANENT ON-LINE MEASUREMENT

The advantages of on-line monitoring and the localization of weak spots in cables are numerous. Information on weak spots and their locations in a cable circuit can now be obtained without circuit-switching actions. Also, the diagnostic tests can be carried out whenever the network owner wishes. The measurement system only has to be installed in the earth leads of the two aboveground termination points in the connected substations in order to perform the task.

For identified critical cable circuits, a system can be installed permanently. These factors reduce maintenance costs and give the network manager time to make the investment decisions required to improve the reliability of the MV network.

PD-OL FIELD TESTING AND EQUIPMENT PRODUCTION

PD-OL has been tested on a 300-m (1000-ft) test cable circuit that was installed in the grounds of the KEMA facility. The principle was proven by intentionally introducing discharges in the cable. Without interrupting electrical service, the location of the problem area was detected within 0.5 m (1.7 ft). This trial has now been successfully conducted using laboratory test equipment on eight additional in-service cables. Figure 2 shows a diagrammatic representation of the PD-OL testing procedure. Figure 3 shows the MV cable splice with a high discharge activity resulting from a broken housing and water ingress.

A Dutch manufacturer has been selected to produce, under license, the first production series of PD-OL measuring instruments, which became available at the beginning of 2006. Three Dutch utilities have already agreed to participate in field trials involving the use of some 60 PD-OL systems.

The location performance of this equipment is expected to be within a few meters for cables up to 4 km (2.5 miles) in length, an accuracy that is sufficient to indicate a faulty cable section or splice. The method provides not only an insight into the weak areas in the cable network, but it also provides valuable information on cables and cable splices. As each cable network has specific characteristics, the measurement equipment will, in some instances, require minor adjustments for local applications.

Network managers within the Netherlands and abroad are already expressing considerable interest this on-line cable diagnostic test.

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Dr. E. Fred Steennis joined KEMA in 1982. He earned his bachelor's degree from the Technical University in Eindhoven and his doctorate in 1989 from the Technical University in Delft, where he researched the degradation mechanisms in the energy cables of the Dutch utilities. In 1991, he received the Hidde Nijland Award for this contribution. With his experience on degradation mechanisms and related test methods, Steennis served as the Dutch representative on the CIGR… Study Committee on High-Voltage Cables in the 1990s and as a member of various international working groups. Steennis is currently a consultant on energy cables, and an author and teacher on the KEMA courses on power cables and the remaining life of power cables. Since 1999, he has been a part-time professor at the Technical University in Eindhoven, where he is teaching and studying diagnostics for power cables. Steennis' main development is that he started PD-OL. fred.steennis@kema.com

Peter van der Wielen studied at the Eindhoven University of Technology, where he received his MSEE degree and finished his doctorate thesis on the on-line monitoring of partial discharges in medium-voltage power cables in 2005. Van der Wielen started research on power cable diagnostics at the Electrical Power Systems Group at Eindhoven University and KEMA, which provides T&D testing services in the Netherlands. His research primarily dealt with aspects related to partial-discharge diagnostics on power cable systems, including cable and substation modeling, pulse propagation, sensors, on-line partial-discharge detection and localization, synchronization and communication. He now works as a consultant/specialist on power cables at KEMA. peter.vanderwielen@kema.com