Power distribution relies on an extensive network of interconnections, many of these being underground power cables. Continuous reliable supply therefore depends on the integrity of these cables, which are ageing progressively from years in operation. While at the HV and EHV level the great impact of an outage warrants extensive redundancy and care to guarantee supply, at the LV level the impact of cable failure is typically limited and redundancy or condition assessment is seen as relatively inefficient. At the MV level, by contrast, impact of an outage can be significant and a balance must to be found between redundancy and monitoring. At the same time, ageing assets and increasing pressure of supply demands proper control over network condition. Moreover, the growing difficulty in finding and training qualified personnel requires operating the MV grid as effectively as possible.
The challenge with underground power cables that are ‘hidden’ underground and difficult to access is that repairs can be cumbersome. Rather, to prevent failures from defective or ageing MV power cable assets, there is a need to rely on measurement practices. Insulation loss (tan-delta) measurement and Partial Discharge (PD) detection are the two main techniques in this regard. Whereas tan-delta indicates general condition of the insulation of the cable under test, PDs usually occur at the location of a defect ahead of most types of failure. This offers the added benefit that PD measurement can pinpoint local defects and, as such, offers a proven technique to assess cable integrity in MV networks.
Such measurements can be performed at intervals, following the concept that cables and other assets in power networks generally show only gradual ageing towards failure. However this concept has been questioned and may not prove valid in the case of all defects. For example, certain types of defect arise due to changing conditions in the environment, maybe in an early stage from factors such as installation error or due to other less predictable causes. Using an offline test, an elevated testing voltage can be applied but might cause damage. This also requires an outage for the cable to be tested and isolated from the network.
On-line PD measurement can give continuous control without having to take a cable out of service. Several kilometers of cable can be monitored at once, passing several intermediate stations. However, sensor placement, signal integrity and noise all need to be carefully considered. Continuous monitoring of PD detection sensitivity can be performed using a signal injected into the monitored cable section, thereby ensuring effective monitoring. Given a properly controlled online monitoring system, continuous and extensive insight can be generated into the condition of a power cable network.
Firstly, this allows learning from PD dynamics and assigning risk based on trending data instead of relying only on instantaneous numbers and thresholds. Second, field experience shows that PD sources can be dynamic in nature and that single ‘sample’ measurements at long-term intervals using offline measurement can yield incorrect assessments. Thirdly, online monitoring offers additional features such as detection and location of permanent and intermittent faults. In sum, online and continuous monitoring brings much greater control over ageing assets while using less resources.
Attend the 2022 INMR WORLD CONGRESS where cable testing expert, Dr. Bart Kruizinga of DNV in the Netherlands will present how such an online PD monitoring system is designed and implemented.