Cable condition assessment and montitoring of online and energized cables using LIRA
1, Sigbjørn Kolberg2
1Wirescan AS, Oppegaard, Norway, 2SINTEF ICT, Oslo, Norway
The LIRA (Line Resonance Analysis) technology is a condition assessment and monitoring tool for a wide aspect of cables. LIRA It is a 100 % non-destructive technology producing valuable information regarding a cables condition based on a wide band impedance spectrum. LIRA presents impedance changes as function of length, a global aging indicator, and a far end termination assessment, all based on the same measurement. LIRA has been utilized on cables in several applications such as offshore wind export cables, downhole umbilicals, interconnectors, distribution grid, and power from shore.
In spring 2016, Wirescan, LIRA technology owner, initiated a two-year R&D-project to develop a method for assessing cables in operation using LIRA. The project is partially financed by Wirescan, the Norwegian Research Council, and commercial partners. SINTEF ICT
The main goal of the project is to develop an online solution producing results correlating to conventional LIRA- measurements. Given the vast number of different cable designs, voltage levels, lengths, etc., the project is, as a start, focusing on getting a viable technology developed for a small selection of applications.
During the first six months of the project, a promising solution was developed in lab environments by simulations, and initial measurements on test samples. In December 2016, a prototype of the solution was tested at SINTEF's high voltage lab in Trondheim with promising results. A full-scale field test is anticipated to be installed early 2017.
This paper describes the general solution and presents the initial test results.
The main principle of the system is to inject a set of discrete frequencies and sense the response with a synchronous detector. The detected signal is then amplified and processed before it is presented as a complex impedance vector. This needs be done without any galvanic connection to the cable, and while the voltage is on and the current is flowing. The impedance vector is input to the LIRA analyzer which is used to assess the cable condition. The proposed solution is under patent application, and no further details can be revealed at this point.
This approach is unique, and will represent a significant improvement to in the field of online condition monitoring.
The initial testing of the system was done in December 2016 at the SINTEF Energy high voltage laboratory in Trondheim. A 195 meter, 240 mm2, cable sample was used as test object. Reference measurements were made with a conventional LIRA measurement with galvanic connection to the cable. No current was flowing in the test cable, since the main goal of the tests were to identify any safety issues with the injector and pickup system.
The initial test results show that the proposed installation method for the injector and pickup introduces no threat to system reliability. This was as expected since the injector and pickup are installed over the outer semiconducting layer. Impedance results show good correlation to the reference measurements.
Further testing is needed to make any specific conclusion on this, but the project is looking to quickly move testing to the field where more data will be produced and a more specific performance of the system will be tested and monitored.
The initial tests confirmed that there are no installation issues related to the injector and pickup, and the tests showed promising measurement data. Results are so far comparable, but field testing is still to be done and it is expected that this testing will produce more valuable information regarding the solution and how it needs improvement.
The first tests of a system allowing LIRA testing of online and energized cables are considered successful showing good correlation between the reference and online measurements.
The main learning objectives of this paper is technical insight into methods for LIRA based online cable condition monitoring, how it works, technical specifications and requirements, limitations, possibilities, benefits, and how such system allows users to gain unique asset integrity insight.