Finding damage and failures in HV cables using passive sensing

Over half of the outage time for high-voltage cables is due to failures that occur in joints and terminations. These weak points are challenging to initially install correctly and can be vulnerable to degradation over time. However, conventional monitoring approaches are sometimes not suitable or are not cost-effective for instrumenting these critical locations. For example, Distributed Temperature Sensing (DTS) does not provide adequate spatial resolution for pinpointing temperature-based anomalies at terminations, and Partial Discharge (PD) monitoring can be impractical, requires specialist analysis, and does not always reveal damage when the system is operating at nominal voltage. Cables are often deployed in underground tunnels or ducts which makes access difficult, unpleasant, and time-consuming for periodic manual inspections.

The authors worked with the Spanish Transmission System Operator (TSO), to retrofit passive sensors to critical locations on a 400 kV underground cable asset. The purpose of the system is to provide visibility of electrical and mechanical parameters at multiple positions distributed along the cable, to deliver early warning of damage and failure modes in these vital locations. The system monitors the following:

• Phase current in each phase of the cable.
• Sheath/screen currents at the start of the cable (at the transition point) and at the joints for all monitored sections of the cable.
• Point temperature on each termination and joint.

In total, 21 current sensors and 12 temperature sensors have been deployed to comprehensively monitor the first three sections of the 400 kV cable. The sensors are entirely passive, requiring no data networks or control power at the sensor locations. For current measurements, split-core current transformers (CTs) have been used, with the CT secondary connected to the passive sensor inputs. Only standard optical fibre is required to couple the remote sensors. A centralised “interrogator” device, installed in the substation at one end of the cable, is connected to the sensor fibre and is able to access synchronised waveform measurements from all sensors nearly instantaneously.

The paper examines all data collected to date, and comments on outcomes from the online monitoring of currents and temperature at multiple locations. For example, as documented in CIGRE B1 report 825 (“Maintenance of HV Cable Systems”), there are clear failure modes in the cross-bonding of sheath connections which can be readily identified from online monitoring of sheath currents with relatively simple heuristics, and the paper highlights how this analysis can be performed continuously and automatically. There are other outliers which can readily be identified, such as a measurement deviating from its normal behaviour, or temperature at one location differing from measurements at other similar locations. Detection of these anomalies will facilitate and optimise maintenance.