The internal pressures in the insulation of two 5-m long pieces of a state-of-the art mass-impregnated non-draining subsea cable have been measured under load current cycling at ambient temperatures ranging from 3 to 37 °C. Thermal expansion of the mass gives in some cases rather extreme internal pressures (>30 bar) and radial pressure gradients (>1.5 bar/mm) in the insulation. This leads to an outwardly directed flow of mass during loading. After a load turn-off, the pressure drops rapidly. Values below 100 mbar were measured in the inner parts of the insulation. The backflow of mass proceeds much slower because the pressure gradient now is smaller. Such a radial redistribution of the mass and the low pressures are assumed to influence the dielectric properties of the insulation as the risk of creating harmful shrinkage cavities is believed to increase. For all the investigated ambient temperature levels the internal pressure in the insulation became around 1 bar or less when isothermal conditions were reached after the load turn-offs. This demonstrates that thermal expansion and contraction of the mass are not the sole mechanisms determining the internal pressures in such cables. Other, still unidentified phenomena are also contributing.

中文翻译：

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电流循环期间大量浸渍的 HVDC 电缆中的内部压力和压力梯度

在 3 至 37 °C 的环境温度下，在负载电流循环下测量了两根 5 米长的最先进的大量浸渍非排水海底电缆的绝缘层的内部压力。在某些情况下，质量的热膨胀会在绝缘材料中产生相当极端的内部压力 (>30 bar) 和径向压力梯度 (>1.5 bar/mm)。这导致在装载期间向外定向的质量流。负载关闭后，压力迅速下降。低于 100 mbar 的值是在绝缘材料的内部测得的。由于压力梯度现在更小，质量的回流进行得更慢。假定质量的这种径向重新分布和低压会影响绝缘体的介电特性，因为据信产生有害收缩腔的风险会增加。对于所有研究的环境温度水平，当负载关闭后达到等温条件时，绝缘体中的内部压力变为大约 1 bar 或更低。这表明质量的热膨胀和收缩并不是确定此类电缆内部压力的唯一机制。其他尚未确定的现象也在起作用。这表明质量的热膨胀和收缩并不是确定此类电缆内部压力的唯一机制。其他尚未确定的现象也在起作用。这表明质量的热膨胀和收缩并不是确定此类电缆内部压力的唯一机制。其他尚未确定的现象也在起作用。