T&D World Magazine

Permanent Monitoring of Medium-Voltage Power Cables

A large portion of the time that electricity consumers are annually without power is caused by failures in medium-voltage power cables and their accessories (often the splices), especially in areas with a high cable density like in cities. The reliability of the energy supply can be considerably improved by locating and repairing weak areas in cables at an early stage. A new measuring technique has been developed that can diagnose and can partial discharge activity a medium-voltage power cable while the cable remains on-line. This system is called PDOL.

Apart from questions concerning the reliability of an electrical grid, the desire to better manage these grids is widespread. Improved reliability is beneficial to the relationship with clients and reduces operating costs. This is especially true for critical cable circuits and almost every grid manager has a number of these. Furthermore, there is a growing concern about the increasing age of the grids and moreover there is considerable pressure to realize savings through the most efficient possible network management, which essentially means only making replacements when strictly necessary.

This inevitably raises questions about which elements of an electrical grid must be replaced and particularly when. The costs to carry out replacements can, however, run into the millions of Euros. In addition, the activities associated with cable replacement create inconvenience in the surrounding area. This implies that the need for replacement must be indisputably established before grid managers start such massive operations.

Interruption minutes
Electricity interruptions as experienced by customers often are related to failures that have their origin in the medium-voltage network, caused by for instance the aging of cables and cable accessories. This is especially the case in cities with hardly overhead lines and thus a high power cable density. Ageing of cables comes from incorrect installation, moisture ingress, corrosion, thermal effects, etc.

Last phase
Aging is causing weak spots that in many cases take time to lead to a failure. To find such weak spots before these fail, off-line diagnostic techniques have been in use for around 10 years that measure and locate these weak spots in the medium-voltage cable network. With such a method, the suspected cable circuit is disconnected from the electrical network. If possible, the energy supply for customers is maintained by rerouting the power through other branches of the grid. Measuring equipment, installed in special vehicles, determines whether there are any weak spots in the cable circuit. An often used method is the measurement of partial discharges. During aging, weak spots exhibit tiny electrical arcs that are measurable. A large number of spontaneous failures in cables and their splices are preceded by such partial discharges. The timely identification of these partial discharges and repair of the weak cable part can prevent many failures and the related inconvenience. This off-line measuring technique has amply proven itself throughout the world during the past decade.

No certainty
The disadvantages of off-line measurements include the switching activities needed (which in some cases even lead to de-energizing customers during this measuring period). Another disadvantage is that partial discharges do not always occur continuously. Because the measurements are made once every several years for tens of minutes, they do not always offer certainty that all of the weak areas will be located on time. In addition, it is impossible to measure the entire medium-voltage network, even if the scope would be limited to the most critical cable circuits, for example just 1 percent of the electricity grid. This means that a cable may still fail at an unexpected location or unforeseen moment.

Online measurement
In 2000, KEMA and the Technical University of Eindhoven in the Netherlands began an initiative that would lead to the permanent monitoring of cables. The PD on-line project for MV power cables was started in 2001, in cooperation with the Dutch grid owners Continuon and Eneco Netbeheer and with support from the Dutch government,. Four years later, the working principle has been proven and the project team is working on additional field tests and the construction of the first production series. The measuring system is called PDOL, which stands for Partial Discharge On-line cable test with Localization.

Detection and Noise
In order to measure partial discharges, in the two substations or Ring Main Units sensitive detection coils are installed at the end(s) of the power cable under test. The measured signals are captured in a small PC 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.

The complexity of the method is found, first and foremost, in the measurement and interpretation of the partial discharges. A discharge from a cable or cable splice is often no larger than a few millivolts. This is almost negligible in comparison with the 10 kV the cable may be carrying. It is extremely challenging to detect the desired signal among the noise from sources such as the switching of thyristors and other disruptions. For this reason, much effort has been directed toward the signal processing methods.

Detection of the partial discharges is only the beginning. These discharges can come from many sources, even from connected transformers or from connected other power cables. Corrective actions such as local repair of weak spots are only possible when the location of these weak spots are known. This is solved by applying two detection coils, one at each cables end (behind the last earth connector, else partial discharge detection is not possible in a simple way). 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 nanoseconds. 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 therefore the location, can be calculated. Processing of the measurements, including display, database storage and interpretation by specialists, occurs at a remote location. Interpretation of the signals is a specialization in itself. With the help of specialists and knowledge rules, grid managers can take action in a timely manner.

Concerning the work of the specialists, one must realize that not every partial discharge signal indicates the same situation. A small partial discharge in a resin insulated splice requires immediate action, while the same partial discharge in an oil insulated splice for instance is of no concern.

Temporary or permanent
The advantages of on-line monitoring and localization of weak spots in cables are numerous. Information on weak spots and their location in a cable circuit can be obtained now without switching actions. Such diagnostic tests can be carried out whenever the grid owner likes it. 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.

First production series
This new on-line measuring technique (PDOL) has been tested on a 300-meter-long test cable circuit that the grid managers installed on the grounds of the KEMA facility. The principle was proven by intentionally introducing discharges in the cable. Without interrupting the electrical service, the location of the problem area was detected within a half meter. In the coming months, field tests are planned of in-service cables. Furthermore, 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.

Eventually, it is expected that weak areas in cables up to 4 km long will be able to be detected with an accuracy of several meters. This is sufficient to indicate a faulty splice or cable section.

Grid managers within the Netherlands and abroad express considerable interest this on-line cable diagnostic test. The method provides not only insight into the weak areas in the cable but also provides valuable information into the cables and cable splices in general. Because each cable network has specific characteristics, the measurement equipment in certain cases must be adapted for the local situation. It is expected that these adjustments are quite minor.

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