Review of High Voltage Direct Current Cables

Increased renewable energy integration and international power trades have led to the construction and development of new HVDC transmission systems.HVDC cables,in particular,play an important role in undersea power transmission and offshore renewable energy integration having lower losses and higher reliability.In this paper,the current commercial feasibility of HVDC cables and the development of different types of HVDC cables and accessories are reviewed.The non-uniform electric field distribution caused by the applied voltage,temperature dependent conductivity,and space charge accumulation is briefly discussed.Current research in HVDC cable for higher operation voltage level and larger power capacity is also reviewed with specific focus on the methodologies of space charge suppression for XLPE extruded cables.

交、直流电场作用下交联聚乙烯中的空间电荷习性（英文）

Polyethylene is one of the widely studied polymeric insulation materials,which have been used extensively for cable insulation.One of the major issues related to polymeric materials is its easy formation of space charge,a high chance to cause electric field distortions.This phenomenon is more significant under high voltage direct current(HVDC)stresses.Space charge can also be observed under high voltage alternative current(HVAC)stresses but with much less intensity due to the limited charge injection period and the effect of charge recombination caused by the constantly variance of the external fields.When considering the situation of an AC voltage combined with a DC offset,a possible scenario in HVDC technology,there was little research on charge dynamics in the insulation in terms of both experimental and simulation work.In this paper,a numerical simulation based a bipolar charge injection/transport model is used to obtain characteristics of space charge in polyethylene under the combined AC and DC high voltage at room temperature.The bipolar charge injection/transport model,which is widely used in HVDC space charge simulation,is applied in the combined conditions.The overall applied voltage,consisted of root mean square(RMS)values of the AC voltage and DC voltage,is kept the same,while the DC component’s voltage ratio and AC component’s frequency are changed respectively,to illustrate their effects on the space charge dynamics within the insulation under combined electric fields.The simulated charge distributions present notable differences when DC offset is increasingly added in,while relatively small differences when AC component’s frequency altering,especially for the cases whose frequency exceeding 0.5 Hz.

关于在电力系统中局部放电对固体绝缘介质的破坏的研究

文章是对于发生在交流电场中在固体绝缘介质的空气间隙的老化进程的实验,通过制造一个在2mm厚度的硅胶样品包含直径1mm的单个的空气间隙,五个样品在12k V的交流电场下加压6小时而后加压到15k V直到有一个样品击穿。在加压过程中,局部放电数据被常规的记录下来。剩下的4个样品将被检查是否有老化的痕迹。初步结果显示,被切割的样品的老化部分有可能是凹凸部分从空气间隙中延伸出来。这篇论文详细介绍这个实验,以及包含获得的老化的样品的照片和详细介绍因为电老化过程对化学成分的变化在硅胶中的影响。

Accelerated thermal ageing on performance of polypropylene-based semiconducting screen for high voltage direct current cable applications-Effect of antioxidants

The potential of using eco-friendly thermoplastic polypropylene(PP)-based insulation for high voltage direct current(HVDC)cable has been widely investigated but much less work on the PP-based semiconducting screen(SC).Considering a long service life(>30 years)under high temperature and high electrical stress is required for typical HVDC cables,and investigations on the effect of antioxidant(AO)concentration and thermal oxidative sta-bility,mechanical,and electrical properties of PP-based SCs have been conducted.It has been demonstrated that an appropriate combination and amounts of AOs are critical for achieving high thermal stability and maintaining the mechanical properties of SC after ageing in a harsh environment(150°C,with Cu,in air,7 days).Although higher amounts of space charges have been observed in SC/PP/SC samples with higher AO concentrations,the impact on space charge behaviours is less after ageing,suggesting that ageing(or operating at high temperature)leads to microstructure evolution in SC and can potentially mitigate space accumulation in PP-based insulating materials.

Investigation on effect of semiconducting screen on space charge behaviour of polypropylene-based polymers for HVDC cables

High voltage direct current(HVDC)power transmission cable is critical for realising sustainability through renewable energy revolution.Eco-friendly thermoplastic polypropylene(PP)-based polymers/nanocomposites are regarded as promising candidates for replacing current thermoset crosslinked polyethylene(XLPE)cables.As an essential component of the extruded HVDC cable for improving conductor/insulation interface and suppressing charge injection to insulation at high DC electrical stresses,developing semiconducting(SC)screens that are compatible with PP-based insulation is of similar importance but has not been well studied yet.This work aims at designing PP-based semiconducting screens and investigating space charge behaviours of SC/PP/SC sandwich specimen to unfold the effect of semiconducting materials,bonding methods,applied DC electric field,and temperature on charge injection,accumulation,transportation,and dissipation in PP-based insulation.Although conventional thermal,mechanical,and low field electrical characterisations demonstrated that all of the developed semiconducting materials meet the performance criteria of commercial semiconducting materials,their space charge and local electric field distribution varied significantly at high DC fields.Compared with the traditional non-bonded configuration used at lab-scale,charge injection was enhanced in hot-pressed SC/PP/SC samples with tightly bonded interfaces,which better reflects the real situation in extruded cables.High temperature further intensified charge injections.Besides,our results also revealed that high temperature and electric field strongly influence charge mobilities and consequently their distribution and local electric field in PP-based insulations.

Effect of organoclay loading on the dielectric properties and charge dynamics of a PP- rubber nanocomposite

The addition of organoclay to a polypropylene-rubber (PP-rubber) blend, primarily introduced to compatibilise the immiscible polymer blend, invokes contrasting dielectric and charge dynamic behaviour depending on the filler loading level. The authors report that at 0.5 wt.% loading, the organoclay decreases the DC conductivity, causes no significant dielectric losses, makes no significant difference to the space charge results compared to the unfilled system, and increases the reproducibility of the breakdown strength results, and hence the reliability of the material. These somewhat surprising results, contrasted by measurements of samples with 2.5 and 5 wt.%, lead us to conclude that trace amounts of organoclay improve the otherwise immiscible polymer blend making organoclay a suitable additive for HVDC applications.

Ageing behaviour of a polyethylene blend:influence of chemical defects and morphology on charge transport

Abstract:The properties of novel cable insulation systems will rely critically upon the morphology of the material.Here,a blend of high and low-density polyethylene(PE)was processed in order to generate three sets of samples with different morphologies.The influence of thermo-oxidative ageing at 120℃ was then considered.The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and were determined by antioxidant consumption and oxygen permeability.Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120°C and,consequently,served to inhibit recrystallisation following ageing.The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology.The electrical conductivity of each system varied non-monotonically with ageing:short times reduced the conductivity;a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold.Despite the pronounced structural differences between the morphologically distinct sets of samples,all exhibited comparable conductivity values beyond this threshold,implying that while charge transport is strongly influenced by chemical factors,crystallinity is relatively unimportant.This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous PE.

Dielectric behaviours of bio-derived epoxy resins from cashew nutshell liquid

In this study,four commercially available bio-derived epoxy systems(extracted from cashew nutshell liquid)were prepared and characterised.The glass transition temperature(Tg),dielectric spectroscopy,DC conductivity and breakdown properties of these epoxy resins were studied.Differential scanning calorimetry(DSC)demonstrated that the T_(g) of the investigated systems ranged from 67 to 122°C.The DC conductivity was very low(<10^(-16) S cm^(-1))and comparable to the conventional dielectrics at room temperature(RT).However,all systems showed a strong temperature dependence of the electrical conductivity and exhibited sharp increase around their respective T_(g).Arrhenius analysis led to activation energy,E_(a),values around 1 eV;higher E_(a) values were observed in systems with a lower T_(g).Dielectric spectroscopy revealed a flat and low response at temperature below T_(g).However,both the real and imaginary permittivity increased with decreasing frequency at mid to low frequencies as the temperatures approached T_(g).The variations of AC breakdown strength of all samples were not statistically significant,but the DC breakdown strength of sample 2503Aþ2002B was higher than the others,which might be due to reduced charge transport in this system.The results indicate that novel bio-derived epoxy systems from renewable sources are potential alternatives for traditional petroleum-based epoxy systems in certain insulation applications.

Investigation of incipient faults in 66 kV oil-filled cable sealing ends

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 pro-gressed 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.

Model to estimate the trapping parameters of cross-linked polyethylene cable peelings of different service years and their relationships with dc breakdown strengths

In this study, an improved trapping/detrapping model was used to simulate the charge dynamics in cross-linkedpolyethylene peelings from different-year aged cables. Injection barrier of trapping parameters was estimated by themodel fitted to experimental data for each type of sample. Moreover, dc breakdown tests were operated on thosesamples. It has been found that the dc breakdown strength of inner-layer samples is the lowest in cable sections withthicker insulation layer taken from high-voltage ac (HVAC) 220 kV service condition, whereas for the cable with thinnerinsulation from HVAC 110 kV, middle-layer samples have worst breakdown performance. This might be explained bythe space charge issues under long-term HVAC condition. More importantly, a clear relationship between estimatedmodel parameters, including injection barrier, trap depth and trap density, with the dc breakdown strength in eachlayer has been reported in this study.