The development of electrical insulation is closely associated with the development of electrical apparatus. Up to about 1925 only naturally occurring products such as asphalt, rubber, mica, cotton thread or fabric were generally used. Mineral oil-impregnated paper was used in transformers, capacitors, and underground cables at voltages up to 69 kV [1]. Then the first synthetic insulating materials, namely phenolic (Bakelite®) and alkyd (saturated polyester) resins, were used in insulating varnishes. Polyvinylchloride was introduced in 1936 to replace natural rubber in underground cable and house wiring, but in spite of tremendous advances in compounding polyvinylchloride to improve its properties, the electrical losses could not be reduced sufficiently. It was restricted to LV applications up to a few kilovolts. A variety of synthetic elastomers was used to replace natural rubber for HV applications, e.g., butyl rubber in the 1940s and ethylene-propylene rubber later on. Epoxy and polyester resins became available around 1945 and were first used for molding and encapsulation of electronic components, then mostly in laminated form as electrical insulation for LV applications [2]. Increased knowledge of the chemistry of insulating materials, and improved processing, then led to the availability of a large number of plastic materials such as polyethylene terephthalate, polycarbonate, polystyrene, low- and high-density polyethylene, polypropylene, polytetrafluoroethylene and fluorinated ethylene-propylene copolymer, suitable in thin film form for capacitor applications. These thin films could work at high electrical stresses or, put more practically, at minimal dielectric thickness appropriate to the capacitor working voltage. Thin films (<;10 μm) of these materials have intrinsic electric strengths in the range 500 to 1,100 V/μm at room temperature, probably 2 to 10 times higher than the average breakdown strength of plastic-film capacitors at the time [3]. Polyethylene-insulated of electrical energy, a statement that is still valid today and certainly will remain valid in the future. The objective of this paper is to give an overview of the research carried out over the last 50 years that has contributed to the development of high-field polymer dielectric materials in a broad sense. During this time thermoplastic polymers have been used in a large variety of electrical systems, e.g., cables, capacitors, transformers, bushings, electrical machines, and accessories operating in a large variety of environments (space, nuclear, thermal, etc.). Progress has been highly nonlinear in time, entangled with progress in material sciences, our understanding of the electrical properties of insulating polymers, and advances in material processing. We emphasize what are, in our view, the main features.

Advances in high-field insulating polymeric materials over the past 50 years

This paper describes very low frequency (VLF) tan delta experiments performed on field-aged and non-aged distribution medium voltage (MV) cable samples. The field aged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of crosslinked polyethylene (XLPE) and water tree retardant cross-linked polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan delta diagnostic measurements at VLF of 0.1 Hz. Results help in clarifying issues that arise when characterizing MV cable insulation by Tan delta diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan delta diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.

Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements

Dielectric response measurement based on the polarisation and depolarisation current (PDC) method is a diagnostic tool commonly employed to the insulation condition assessment of power apparatus. In this paper, experimental studies are carried out on short sections of service-aged XLPE cable. Daniel's normalisation is applied to estimate the difference between the permittivity under DC excitation and that at optical frequency. Physical explanations of various polarisation processes at low and high frequencies are interpreted through the clustering model. Based on transforming the PDC currents into the frequency domain equivalents, dissipation/loss factor (tan δ) measurement and modeling over the range of frequency from very low to power frequency are presented. The analysis reveals the relationship between individual loss factors and the overall loss factor. Also, measurements of the activation energy and testing under varying electrical stresses and ambient conditions provide further information for interpretation.

Dielectric response study of service-aged XLPE cable based on polarisation and depolarisation current method

In any sustainable electrical systems, various high-voltage (HV) power apparatuses such as electrical cable, transformer etc. play a pivotal role to transfer electrical energy from the generation end to the consumer end. The degradation due to electrical stress is very unlikely to avoid in operation. Prominently, the insulation system is mostly expected to be strong enough, and its degradation always provides a constant threat to the reliable operation. Moreover, the continual deterioration can gradually lead to the complete breakdown of HV equipment. To assess the overall dielectric properties, the regular monitoring/diagnosing of electrical insulation has a paramount importance in power systems. One of the potential tools for diagnostic assessment is the tan-delta (tan δ
) measurement. Considering different aspects at various frequencies, this study elucidates a comprehensive review of tan δ
 measurement. First, a detailed review and explanation of multiple challenges associated with dielectric loss measurement are presented. Various dielectric losses, their contribution to tan δ
, measurement techniques and their comparisons are also discussed. This study further reviews the analogy of different physical dipolar theories, modelling and a brief analysis of commercially available measurement instruments.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-smt.2018.5597

Diagnostic challenges in dielectric loss assessment and interpretation: a review

The main contribution of this paper is to investigate the correlation between tan delta diagnostic measurements at 0.1 Hz (Very Low Frequency-VLF) and VLF breakdown performance for medium voltage (MV) cable samples through a laboratory test program. The cable samples used are a set of 15 kV, Cross-linked Polyethylene (XLPE), unjacketed cables removed from the field, the same service area, and having experienced similar operating conditions for almost four decades. The test program includes tan delta measurements at different voltage levels and a subsequent VLF extended step-withstand test. The VLF step test allows the evaluation of risk of failure during VLF tan delta testing and assessment of the ultimate performance of the cables. The tan delta diagnostic measurements are represented by the tan delta value and tip-up, which are considered the classical metrics. However, the paper also suggests the use of a new additional diagnostic feature that takes into account the scatter in the tan delta measurements for a particular test voltage level.

Correlation between tan δ diagnostic measurements and breakdown performance at VLF for MV XLPE cables

This paper reports the results from the condition assessment of 12- and 24-kV cross-linked polyethylene (XPLE) cables using a technique based on dielectric spectroscopy initially developed at KTH in Sweden. The work aims to examine whether the method could detect water tree degradation for the second generation medium voltage (MV) cables with long, but not bridging, water trees. While the overall cable condition was better than expected for second generation XPLE cables, water trees were found in most of the selected cables. The diagnostic method based on the measurement of the dielectric response could only detect water tree degradation in the examined second generation cables when the water trees bridged the insulation wall. Condition assessment above service stress may, in some cases, be required to detect bridging water trees. The results indicate that there is a correlation between the voltage level and the breakdown voltage of the cable. This can be used as a diagnostic criterion for this group of cables.

B. Holmgren

Papers

Condition assessment of 12- and 24-kV XLPE cables installed during the 80s. Results from a joint Norwegian/Swedish research project