Learnings

Outcomes

There were several conclusions from the investigation which include, but are not limited to:

1 Compact HV Supplies for Cable Commissioning Tests

One of the main cable tests is a voltage withstand test. This test aims to subject the cable to enough stress to reveal potentially life-limiting defects. The power supply requirements of this test are very demanding. Finding lower power alternatives to complete this test would be very useful; particularly in the North of Scotland where site access does not always
allow for the large vehicles that transport the physically large Transmission-grade test sets. For alternatives to resonant HV supplies for voltage withstand testing, Very Low Frequency (VLF) and Damped AC (DAC) remain the only options capable of generating the voltages required for cables rated at 132 kV and above. However, effective use of either is prevented by the compromises inherent in the two approaches, namely:

A) Compliance with cable manufacturers’ strict limitations on applied voltages and charging times that are based on avoiding any risk of space charge injection (regarded as insulation ageing). This renders testing significantly less rigorous than a 1- hour AC voltage withstand at near power frequency and could occasionally result in cables with incipient insulation defects
passing the test.

B) Making the alternative tests more rigorous (extending their duration and/or increasing the test voltage) to properly stresstest the insulation. This approach makes it likely that the manufacturer would consider its warranty has been voided in the event of failure during the test or within the initial in-service warranty period.

An important recommendation is that the intended method of voltage withstand testing should be considered from the outset of planning for a new cable installation. This will avoid unforeseen complications arising later when the cable manufacturer becomes the arbiter of what is permitted.

2 Partial Discharge (PD) Detection

PD signals propagating many km through a cable, and a number of joints, will experience significant levels of attenuation that are difficult to quantify. In the absence of defined standards to control the efficacy of PD detection in the field, it is important to have concrete evidence from the test service provider concerning the robustness of their test procedure
and their validation of its operation prior to testing. The contractor monitoring for PD during commissioning tests should be asked to provide documented evidence of the absence of PD when a cable has passed the voltage withstand test. This ought to provide useful reference information including the background noise levels, any interference present and the
duty cycle of monitoring.

3 Cable Impedance Scanning

This technique is carried out on non-live cables and uses relatively small test voltages to detect and, most importantly, locate damage or deterioration in parts of the cable. Measurements of this kind (such as Time Domain Reflectometry (TDR)) make use of the transmission line properties of screened HV cables to detect variations in characteristic
impedance as a function of distance from the test terminals. Known discontinuities in cable impedance (such as joints and terminations, where the coaxial geometry is disrupted) will cause characteristic signal reflections at distances that can be compared with their known positions along the cable. Additional (unexpected) reflections due to mechanical damage or
changes in impedance caused by differences in dielectric properties at different parts of the cable can provide diagnostic information. The most important aspect is to look for changes between the present scan and a previous scan to determine at what position(s) along the cable any changes might have taken place. This is an emerging approach that may merit further investigation, particularly on new cables.

Lessons Learnt

From the final report, recommendations with the potential for further innovation and exploration of the project findings are as follows:

1. Trapped DC Voltages on AC Cables

During the TACAMA project we were made aware of the following potential issue with service-aged cables, which relates to the topic of space charges in cross-linked polyethylene (XLPE) insulation that causes manufacturer concerns about the use of Very Low Frequency (VLF) and Damped AC (DAC) for commissioning tests. When a cable is switched out of service, a DC voltage may persist (stored on the cable capacitance). This trapped charge may damage the cables and their accessories. We found minimal published works on this phenomenon or its potential effect on XLPE cables. This issue may merit further research.

2. Energy Harvesting for Autonomous Sensing

Various cable sensor technologies were discussed during the project. An emerging type of technology is sensors that meet their (small) power requirements from the electrical circuit they’re connected to, rather than having a dedicated power supply. This would allow sensors to be installed in remote locations with wireless comms capabilities to transmit data. The
recommendation is that cable installations could be ‘future proofed’ by ensuring they have the facilities to retrofit sensors (as well as the means to power future sensors). This may only involve simple modifications to the physical configuration of connections at joints, for example.

3. Other Industry Standards Under Development

There are several other industry standards which are likely to expand upon this area of investigation. There is an opportunity for shared learning between these groups and network operator cable teams. These include, but are not limited to:
• It is expected that the IEEE Standards Committee will initiate a review of its VLF test standard 400.2.
• CIGRE Study Committee B1 is working on an 8-year effort to create a report (CIGRE B1.38) which will explore: “After Laying Tests on AC and DC Cable Systems with New Technology”.