Engineering Analysis: Why does VDE 0250 mandate an inner semi-conductive layer for all rubber cables rated 6kV and above?
Published by: Anhui Feichun Special Cable Co., Ltd.
Specialists in Medium Voltage Electrical Field Stress Control
1. Executive Summary: Controlling the Invisible Stress
2. The Physics of Failure: The “Strand Effect”
Flexible cables use stranded conductors (Class 5). This surface is not smooth; it is bumpy, resembling a bundle of cylinders.
1. Air Voids: If insulation were applied directly to bare copper, tiny air pockets would remain between the strands and the rubber.
2. Field Concentration: In electrostatics, electric fields concentrate at sharp points (the curve of each copper strand).
3. Ionization: Air breaks down (ionizes) at a much lower voltage (~3 kV/mm) than rubber insulation (~20+ kV/mm). At 6kV, the electric field stress in these air pockets causes the air to “flash” internally (Partial Discharge).
柔性电缆使用绞合导体（第5类）。这个表面不光滑；它是凹凸不平的，类似于一束圆柱体。
1. 气隙： 如果绝缘层直接施加在裸铜上，微小的气穴将残留在股线和橡胶之间。
2. 电场集中： 在静电学中，电场集中在尖点（每根铜股线的曲线）处。
3. 电离： 空气的击穿（电离）电压（~3 kV/mm）远低于橡胶绝缘（~20+ kV/mm）。在 6kV 时，这些气穴中的电场应力导致空气内部“闪络”（局部放电）。
| Scenario (场景) | Result (结果) |
|---|---|
| Direct Insulation (No Semi-Con) 直接绝缘（无半导电层） | Electric field creates ozone ($O_3$) in air gaps -> Ozone attacks rubber -> “Treeing” channels form -> Breakdown. 电场在气隙中产生臭氧 ($O_3$) -> 臭氧攻击橡胶 -> 形成“树枝状”通道 -> 击穿。 |
| With Inner Semi-Con 有内半导电层 | Semi-con layer fills all gaps -> Creates smooth cylindrical surface -> Zero Potential Difference between copper and semi-con -> No Discharge. 半导电层填充所有间隙 -> 创建光滑的圆柱表面 -> 铜和半导电层之间零电位差 -> 无放电。 |
3. The Solution: Creating a Faraday Cage
Key Function: It acts as a Faraday Cage around the rough copper strands. It pushes the electric field out to its smooth, circular surface. Since there are no air gaps between the semi-con layer and the insulation (because they are bonded), there is no place for partial discharge to start. 内半导电层本质上是一种填充炭黑的橡胶化合物。它的导电性足以与铜导体处于相同的电位，但又足够光滑，可以与绝缘层完美结合。
关键功能： 它在粗糙的铜股线周围充当法拉第笼。它将电场推向其光滑的圆形表面。由于半导电层和绝缘层之间没有气隙（因为它们是粘合的），因此没有局部放电开始的地方。
4. Manufacturing Excellence: Triple Extrusion
At Anhui Feichun Special Cable Co., Ltd., we utilize a Triple Co-Extrusion Process for our MV cables (e.g., (N)TSCGEWÖU).
- Step 1: Inner Semi-Conductive Layer.
- Step 2: 3GI3 Insulation (EPR).
- Step 3: Outer Semi-Conductive Layer.
All three layers are extruded simultaneously in one head (“Wet-on-Wet”). This ensures they fuse together chemically on a molecular level, guaranteeing there are absolutely no air voids at the interfaces. Older methods of applying layers separately (“Wet-on-Dry”) are prone to interface contamination and are not used for high-quality VDE cables.
这三层在一个机头中同时挤出（“湿对湿”）。这确保了它们在分子水平上发生化学融合，保证界面处绝对没有气隙。分层施加的旧方法（“湿对干”）容易导致界面污染，不用于高质量 VDE 电缆。
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