Harmonised test for the measurement of residual methane in insulating materials

Introduction

At present there is no standardised method for measuring the methane content of a power cable and there are significant differences between the various methods currently used in the industry. In 2012 a technical brochure (TB 501) on the basic principles to determine methane content in medium and high voltage cables was released [B2]. The scope of this technical brochure was to review state of the art and evaluate the precision of the available methods. The main recommendation from this work was to develop a new optimised method based on gas chromatography (GC) for the measurement of inflammable gasses in peroxide cured power cable insulation systems.

This report continues the work which led to the publication of TB 501 and details a harmonised test method to be used for determining the residual content of methane in cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR) or other similar products used in the insulation system of power cable systems.

The scope of the current work is detailed in the following Terms of Reference (TOR):

• Define/optimise a harmonised test procedure for the measurement of residual inflammable gases in insulating materials.
• Specify details of the test procedure such as the sampling method, equipment and equipment calibration, measurement method, and evaluation of method precision.
• If possible, a rationale will also be given to explain the basis for the chosen parameters.

While methane can be produced in significant amounts, other inflammable gasses, such as ethane, are generated in trace amounts only and most labs do not routinely quantify them. Therefore, only methane gas was considered in this work, although in principle the recommended method is expected to perform equally well with other gasses.

Harmonised method for methane measurements

A method for sample preparation has been defined with primary focus on robustness, repeatability and safety. Many parameters such as sampling position, cutting procedures, sample size, time and temperature have been considered and investigated experimentally as needed.

Furthermore, recommendations are provided on the equipment used for sampling, especially sample flask, septum and syringe. Finally, the GC instrument and its calibration are discussed, and some recommendations and suggestions are provided.

The method is divided into three principal steps which are described in respective sub-chapters. The first step is sample preparation where the cable insulation is cut into pieces and enclosed in a flask equipped with septum. This step takes place at the cable manufacturing site and relies on quick and precise execution in order to get a representative sample. The sample-containing flask can be stored for some time or shipped to an external lab for analysis without significant loss of methane. The second step of the method concerns calibration of the GC equipment. It is demonstrated in the TB that the calibration can be performed in a few different ways with equivalent results. Calibration gas can be obtained by diluting pure methane locally, but many labs choose to acquire certified gas mixtures in a series of suitable concentrations directly from their gas suppliers. The measurement of methane in the sample flask is the final step and includes the extraction of methane inside the flask by means of heat treatment. Significant pitfalls have been identified in the above-mentioned steps and by avoiding these it is possible to achieve reliable and repeatable measurements with accurate results that are reproducible across different analytical labs.

Description of Technical Brochure

Chapter 1 is the introduction to this TB and describes the background of this WG, its TOR and scope.
Chapter 2 outlines a harmonised test method for the measurement of methane in power cable insulation systems.
Chapter 3 describes the round robin testing performed by this working group
Chapter 4 describes the individual test methods used by the round robin participants
Chapter 5 contains conclusions and recommendations for further work