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Abstract This paper reports on the investigation of incipient fault mechanisms of a widely used family of Cable Sealing Ends (CSEs) which use an earthed metallic clip on top of the stress cone to fasten the cable termination stress grading. Finite element simulations indicate that the clip is causing localised electric field enhancement which could also cause impurities in the oil to move towards the clip under the influence of the dielectrophoretic force. Accelerated ageing tests of stress cones in the laboratory have corroborated the simulation results, with the high electric field causing degradation of the semi‐conducting tape underneath the clip as well as initiation of tracking on the surface of the stress cone originating from the edges of the clip. Partial discharge (PD) testing using a bespoke test rig in the high voltage laboratory has shown that incipient faults originating from the ageing of the solid CSE materials can only be detected after surface tracking has progressed substantially. Even then, the intermittent nature of PD activity can pose a risk to the accurate condition assessment of CSEs if continuous monitoring is not employed and if the ultra‐high frequency method is used on its own.

HVDC circuit breakers are of increasing importance, as multi-terminal high voltage DC (HVDC) transmission becomes a commercial reality. Multiple HVDC breaker technologies have been developed, and are starting to appear as proof-of-concept installations on real networks. Information about them is however distributed widely in the literature. This article describes the underlying challenges, the leading candidate solutions, and discusses the requirements and construction of the component sub-assemblies. Its goal is to provide a comprehensive overview of the field.

The aim of this study is the presentation of the results of an in-lab comparative study of electrical and thermal monitoring of artificially polluted, HTV-textured silicone rubber insulators, with different pollution levels. This work is a preliminary study of an in-situ monitoring of 400 kV SiR textured in a polluted environment. The results showed that the rms leakage current magnitude and pulses, and the average dissipated power depended on the pollution levels and the dry-bands formation. The discharge activity and their nature are governed by the pollution level and the voltage. A differentiation and a quantification between dry-band discharge onset and dry-band arc inception is highlighted.

Multi-terminal voltage-sourced converters (VSC) high-voltage direct current (HVDC) transmission system is expected to play a vital role in future power systems. Compared with ac power transmission, dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters. Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built. The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters, detailed equivalent models for the modular multi-level converters, detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables. Using such high-fidelity system models, a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted. This is the first comprehensive study that includes pre-emptive circuit breaker operation. The results presented in this study underline the benefits of such a detailed treatment of the breaker, and of considering it as part of a fast power electronics system rather than isolated dc equipment. The study identifies the best existing fault detection method and tests it extensively. In order to further improve post-fault system recovery response, which is a key but often neglected part of previous studies, a novel bump-less transfer control has been implemented in the converters.