Thermal limit of XLPE insulation: is 90 still the magic number?

Abstract

The conventionally accepted maximum operating temperature of XLPE insulated power cables in “normal operation” is 90°C as per IEC 60840 and IEC 62067. This limit has been in place for many decades, while the technology has continued to evolve and mature from the invention of XLPE in 1963, through to the highly robust and reliable cables being manufactured today. As time passes, the original rationale behind such limits can become obscured. The aim of this paper is to highlight the various reasons behind the existing limits, to consider them again in the light of present-day knowledge and experience and assess whether the technology has more to offer going forward.

The IEC standards specify only a single option for the maximum conductor temperature, with the same value used for both continuous and short-term operation. Looking to other standards, it is common to allow a short period of operation above the 90°C specified by IEC. For example, in the US market an emergency operating temperature of 105°C is permitted, and similar allowances are made in standards in Japan. These elevated operating temperature regimes were originally put in place with reference to the requirements of grid operators, rather than the performance limit of the cable system itself. This raises a number of questions, such as: could the cable actually tolerate elevated temperatures for longer periods? What are the actual limits, and what physical phenomena are involved in setting these limits? This becomes particularly interesting for systems which might be required to support high loads, but for limited and well-defined time periods, such as those connected to sources of renewable power generation.

In setting the permissible operating temperature, there are many considerations to make. Thermal ageing of the insulation material itself is only one of these, in reality comprising a number of linked issues from long term effects on the polymer structure itself to the depletion of anti-oxidants. Thermal and mechanical impacts of elevated temperatures are often coupled with the electrical behaviour of the system, especially at interfaces; a classic example here is the impact of thermal expansion and contraction on the interface pressure, where the electrical strength of interfaces can be reduced if the pressure drops beyond a certain threshold. All of these factors need to be considered holistically when determining the permissible operating temperature of the cable circuit.

Our paper attempts to map the range of factors of relevance in assessing if conductor temperature higher than 90°C is feasible for more than the limited “emergency operation” defined in some standards. Crucially we seek to highlight what may have changed over the decades since the original 90°C limits were set and propose the steps which could be taken to expand the functionality of XLPE insulated cables into the future.

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Identifiers

ORCID for James Pilgrim: orcid.org/0000-0002-2444-2116

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

ORCID for Paul Lewin: orcid.org/0000-0002-3299-2556

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