Reliable energy transport is fundamental to both onshore and offshore power infrastructure and therefore quality control of newly installed as well as service aged cable connections is a matter of great importance for any transmission grid operator (TSO). One of the key questions in this regard is how best to detect possible poor workmanship-related defects in new cable circuits in a sensitive yet non-destructive manner. Another is how best to perform non-destructive diagnostics of existing cable circuits to determine their actual condition. Paul Leufkens, a power industry expert, has spent more than 20 years working for consulting as well as testing companies and has also directed product development in both cables and switchgear. His paper and presentation at the 2019 INMR WORLD CONGRESS will focus on use of DAC for after-laying testing and diagnosis of all types of transmission cables, based on years of experience collected from power grids across the globe.
In the IEC standards for power cables up to and above 150 kV operating voltage, testing protocols after installation are limited to manufacturer minimum recommendations. As such, these do not necessarily meet the need to ensure failure risk during operation is kept as low as possible. Specifically, there are a number of critical aspects to consider in responsible operation and asset management of transmission cables. For example, testing newly installed cable systems must be able to reveal:
1. Manufacturing related defects → less likely to occur due to the high level of quality control in factories;
2. Accessories parts delivery problems → more probable due to increasing diversification in supply chains;
3. Installation related defects → highly probable due to diversification in the installation supply chains as well as practical difficulties when installing HV cable systems on or offshore.
Similarly, maintenance and diagnostic testing of cable systems already in operation should allow an estimate of:
1. Operational damage and electrical/thermal overstress → cannot be neglected due to the influence of transients and overvoltage;
2. Ageing processes → will depend on many operational and local factors such as presence of installation defects, load constrictions;
3. Remaining life → goal of asset managers is to keep CAPEX and OPEX at an optimal level.
Unfortunately, traditional testing companies, contractors and also some TSO themselves refer mainly to IEC procedures to ensure quality. But these standards, i.e. IEC 60840 and IEC 62067, were introduced 30 years ago and have high manufacturer influence. For example, they extensively discuss aspects of factory testing while mentioning only basic tests after-installation. Moreover, no guidelines are provided in regard to maintenance of cable circuits or maintenance/diagnostic testing. In particular, IEC 60840 and IEC 62067 only recommend a Go/No-Go decision as a result of a breakdown by one of those tests:
• Soak test with Uo at 24 hours; or
• After-installation test with 1.73 Uo or 2.0 Uo for cables with rated voltages up to 150 kV;
• After-installation test with 1.7 Uo, 1.4 Uo or lower for cables with rated voltages above 150 kV.
Unfortunately, overvoltage based destructive tests are not ideal to verify presence of possible insulation weak spots in newly installed sections of cable. In the case of an installation error, breakdown will occur only in the case of an extremely serious defect. This is a rare situation. But even if no breakdown occurs, the presence of defects in accessories cannot be excluded. In fact, breakdowns during testing seldom occur and the major reason for cable failure after installation (i.e. in some 30% of all failures) are defects in or at the interface of joints and terminations. Most of these installation defects result from poor workmanship. Moreover, about 80% of insulation defects in power cables are visible only using advanced diagnostics, e.g. sensitive partial discharge detection.
Considering the focus on reliability, cable failures can be prevented if TSOs maintain and update testing procedures to reflect newest industry developments, such as recent IEEE standards that were developed as a result of co-operation between power suppliers, manufacturers and testing companies. In combination with selected IEC documents, these represent the state-of-the-art in non-destructive methods for both after-installation and maintenance testing and diagnostics.
At the 2019 INMR WORLD CONGRESS, Mr. Leufkens will review this important topic.
Register Now at: www.inmrworldcongress.com