The transition from low voltage to medium voltage cable design represents a fundamental shift in electrical engineering requirements. In Australian electrical infrastructure, cables are classified based on their voltage rating, with 1.1 kilovolts serving as a critical boundary between two distinct design philosophies. Cables operating at or below 1.1kV are considered low voltage applications and typically do not require the sophisticated screening systems mandated for higher voltages. However, once voltage ratings exceed this threshold and enter the medium voltage range starting at 1.9/3.3 kilovolts, Australian Standards AS/NZS 1429.1 imposes strict requirements for semi-conductive screening systems.¹

从低压到中压电缆设计的过渡代表了电气工程要求的根本转变。在澳大利亚电气基础设施中,电缆根据其额定电压进行分类,1.1 千伏是两种不同设计理念之间的关键界限。额定电压等于或低于 1.1kV 的电缆被视为低压应用,通常不需要更高电压所要求的复杂屏蔽系统。然而,一旦电压等级超过此阈值并进入从 1.9/3.3 千伏开始的中压范围,澳大利亚标准 AS/NZS 1429.1 对半导电屏蔽系统提出了严格要求。

This voltage threshold reflects the physics of electrical stress distribution within cable insulation. At lower voltages, the electric field stress remains manageable even without specialised screening, and the risk of insulation breakdown through partial discharge phenomena is minimal. As voltage increases beyond 1.1kV, however, the electric field intensities within the cable insulation rise dramatically, creating conditions where uncontrolled stress concentrations can initiate progressive insulation degradation and eventual failure.²

这一电压阈值反映了电缆绝缘内电应力分布的物理特性。在较低电压下,即使没有专门的屏蔽,电场应力仍然可控,通过局部放电现象导致绝缘击穿的风险也很小。然而,当电压超过 1.1kV 时,电缆绝缘内的电场强度急剧上升,造成不受控制的应力集中可能引发渐进性绝缘劣化并最终导致故障的条件。

The primary purpose of semi-conductive screens in medium voltage cables is to establish and maintain a uniformly divergent electric field throughout the cable’s insulation system. In cables operating above 1.1kV, the conductor surface typically consists of multiple strands wound together, creating an inherently irregular geometry with numerous peaks, valleys, and air pockets between individual strands. Without a semi-conductive conductor screen, these geometric irregularities would create localised regions of extremely high electric field stress at the strand tips and edges.³

中压电缆中半导电屏蔽层的主要目的是在整个电缆绝缘系统中建立并保持均匀发散的电场。在工作电压高于 1.1kV 的电缆中,导体表面通常由多股绞合在一起组成,形成固有的不规则几何形状,在单股之间存在许多峰、谷和气隙。如果没有半导电导体屏蔽层,这些几何不规则性将在股线尖端和边缘产生极高电场应力的局部区域。

The semi-conductive conductor screen smooths these irregularities by providing a continuous, equipotential surface that makes the voltage on the inside boundary of the insulation completely uniform. This is accomplished through the use of carbon black particles dispersed within a polymer matrix, creating a material with carefully controlled electrical resistivity that is neither fully conductive nor fully insulating. By filling in the gaps between conductor strands and eliminating air voids at the conductor-insulation interface, the conductor screen ensures that the electric field emanates radially from a smooth, cylindrical surface rather than from multiple irregular points.⁴

半导电导体屏蔽层通过提供连续的等电位表面来平滑这些不规则性,使绝缘内边界上的电压完全均匀。这是通过在聚合物基质中分散碳黑颗粒来实现的,创造出一种具有精心控制的电阻率的材料,既不完全导电也不完全绝缘。通过填充导体股线之间的间隙并消除导体-绝缘界面处的空气空隙,导体屏蔽层确保电场从光滑的圆柱表面径向发射,而不是从多个不规则点发射。

The insulation screen serves the complementary function of containing the electric field generated by the energised conductor entirely within the primary insulation layer. This outer semi-conductive screen, applied over the insulation surface, provides a second equipotential surface that acts as the outer boundary of the dielectric field. When properly grounded through the cable’s metallic screen, the insulation screen ensures that all electric field lines terminate at a known, controlled potential rather than extending into surrounding materials or air spaces.⁵

绝缘屏蔽层具有互补功能,即将带电导体产生的电场完全限制在主绝缘层内。这种施加在绝缘表面上的外部半导电屏蔽层提供了第二个等电位表面,作为介电场的外边界。当通过电缆的金属屏蔽层适当接地时,绝缘屏蔽层确保所有电场线终止于已知的、受控的电位,而不是延伸到周围材料或空气空间。

This field containment becomes increasingly critical as voltage levels rise above 1.1kV. At medium voltages, any electric field extending beyond the designed insulation boundaries can induce stress in materials that were not engineered to withstand such conditions. Braids, tapes, water-blocking layers, and other cable components adjacent to the insulation must be excluded from the active dielectric field to prevent their degradation and to maintain predictable electrical performance over the cable’s service life.⁶

随着电压水平上升至 1.1kV 以上,这种场控制变得越来越关键。在中压下,任何延伸超出设计绝缘边界的电场都可能在未设计用于承受此类条件的材料中产生应力。必须将与绝缘相邻的编织层、带材、阻水层和其他电缆组件排除在有效介电场之外,以防止其劣化并在电缆使用寿命内保持可预测的电气性能。

Partial discharge represents one of the most significant degradation mechanisms affecting medium voltage cable systems. Partial discharge occurs when the electric field within a microscopic void, air gap, or defect exceeds the breakdown threshold of the gas occupying that space, typically around 3 kilovolts per millimetre for air at atmospheric pressure. In cables rated above 1.1kV, operating voltages are sufficient to initiate partial discharge activity within any voids present at conductor-insulation or insulation-screen interfaces.⁷

局部放电是影响中压电缆系统的最重要的劣化机制之一。当微小空隙、气隙或缺陷内的电场超过占据该空间的气体的击穿阈值(通常在大气压下空气约为每毫米 3 千伏)时,就会发生局部放电。在额定电压高于 1.1kV 的电缆中,工作电压足以在导体-绝缘或绝缘-屏蔽界面处存在的任何空隙内引发局部放电活动。

Once initiated, partial discharge creates a cascade of destructive effects. Each discharge event transfers high-energy ion bombardment onto the insulation surface, breaking molecular bonds within the polymer structure. Chemical reactions between the polymer and ionisation by-products such as ozone progressively erode the insulation material. While individual partial discharge pulses may involve only picocoulombs of charge, their repetitive nature means they occur thousands of times per second during normal operation, leading to cumulative damage that can cause complete insulation failure within months to years.⁸

一旦启动,局部放电会产生一系列破坏性影响。每次放电事件都会向绝缘表面传递高能离子轰击,破坏聚合物结构内的分子键。聚合物与电离副产物(如臭氧)之间的化学反应会逐渐侵蚀绝缘材料。虽然单个局部放电脉冲可能仅涉及皮库仑的电荷,但其重复性质意味着在正常运行期间每秒发生数千次,导致累积损坏,可能在数月至数年内造成完全绝缘故障。

Semi-conductive screens directly address this failure mechanism by eliminating air voids at critical interfaces. The extruded conductor screen fills all gaps between conductor strands, ensuring intimate contact between conductor and insulation with no air pockets where partial discharge could initiate. Similarly, the bonded insulation screen creates a void-free interface at the outer boundary of the insulation layer. This is why AS/NZS 1429.1 specifies that both conductor and insulation screens must be applied through co-extrusion with the insulation layer in a single, continuous manufacturing process, creating molecularly bonded interfaces that cannot separate during cable installation or operation.⁹

半导电屏蔽层通过消除关键界面处的空气空隙直接解决这一故障机制。挤出的导体屏蔽层填充导体股线之间的所有间隙,确保导体和绝缘之间的紧密接触,没有可能启动局部放电的气隙。同样,粘合的绝缘屏蔽层在绝缘层的外边界处创建无空隙界面。这就是为什么 AS/NZS 1429.1 规定导体和绝缘屏蔽层都必须通过与绝缘层在单一连续制造过程中共挤出来施加,创建在电缆安装或运行期间不会分离的分子粘合界面。

The electric field within a medium voltage cable has both radial and tangential components that must be carefully managed to prevent insulation failure. In an ideal coaxial cable geometry, the electric field should be purely radial, varying inversely with distance from the cable centre according to the relationship E equals voltage divided by radius times the natural logarithm of the outer radius over inner radius. This radial field distribution is contained entirely within the insulation, with field lines running perpendicularly from the inner conductor screen to the outer insulation screen.¹⁰

中压电缆内的电场具有径向和切向分量,必须仔细管理以防止绝缘故障。在理想的同轴电缆几何形状中,电场应该是纯径向的,根据关系式 E 等于电压除以半径乘以外半径与内半径比值的自然对数,与电缆中心的距离成反比变化。这种径向场分布完全包含在绝缘内,场线从内导体屏蔽层垂直运行到外绝缘屏蔽层。

Problems arise when the semi-conductive screens are cut back during cable termination or jointing operations. At the point where the insulation screen is removed, the electric field is no longer purely radial. A tangential component develops along the insulation surface, with equipotential lines crowding together at the screen cutback edge. This concentration of equipotential lines indicates extremely high tangential electric stress, which can exceed the breakdown strength of air or surface contaminants. Industry practice generally requires that tangential stress be kept below 2.5 kilovolts per centimetre to avoid surface discharges and tracking.¹¹

当在电缆终端或连接操作期间切除半导电屏蔽层时,会出现问题。在绝缘屏蔽层被去除的点,电场不再是纯径向的。沿绝缘表面产生切向分量,等电位线在屏蔽层切除边缘聚集在一起。这种等电位线的集中表明切向电应力极高,可能超过空气或表面污染物的击穿强度。行业实践通常要求切向应力保持低于每厘米 2.5 千伏,以避免表面放电和跟踪。

The semi-conductive screens enable proper stress control at cable terminations and joints through the use of stress cones or stress control materials. These accessories work in conjunction with the factory-applied screens to gradually redistribute the electric field over a longer distance, separating the equipotential lines and reducing peak stress values. Without the well-defined screen edges provided by properly manufactured semi-conductive layers, effective stress control would be impossible to achieve, and cable accessories would fail prematurely through electrical tracking and discharge activity.¹²

半导电屏蔽层通过使用应力锥或应力控制材料在电缆终端和接头处实现适当的应力控制。这些附件与工厂施加的屏蔽层配合使用,在更长的距离上逐渐重新分配电场,分离等电位线并降低峰值应力值。如果没有由适当制造的半导电层提供的明确定义的屏蔽层边缘,就无法实现有效的应力控制,电缆附件将因电跟踪和放电活动而过早失效。

Australian Standards specify precise requirements for semi-conductive screen materials and their application methods. The semi-conductive compound consists of an ethylene-based polymer matrix into which carbon black particles are dispersed to provide controlled electrical conductivity. The concentration and dispersion quality of carbon black is critical, as it must provide consistent resistivity throughout the screen layer while maintaining the mechanical properties necessary for cable flexing and installation.¹³

澳大利亚标准对半导电屏蔽材料及其应用方法规定了精确要求。半导电化合物由乙烯基聚合物基质组成,其中分散有碳黑颗粒以提供受控的导电性。碳黑的浓度和分散质量至关重要,因为它必须在整个屏蔽层中提供一致的电阻率,同时保持电缆弯曲和安装所需的机械性能。

For cables meeting AS/NZS 1429.1 requirements, the conductor screen and insulation screen are applied through a triple extrusion process where all three layers (conductor screen, primary insulation, and insulation screen) are co-extruded simultaneously. This manufacturing approach creates molecularly bonded interfaces between layers, ensuring that no delamination can occur during cable bending, temperature cycling, or mechanical stress. The bonding between layers is so complete that the insulation screen cannot be stripped away cleanly, which distinguishes bonded screens from the strippable screen designs used in some other international standards.¹⁴

对于满足 AS/NZS 1429.1 要求的电缆,导体屏蔽层和绝缘屏蔽层通过三重挤出工艺施加,其中所有三层(导体屏蔽层、主绝缘和绝缘屏蔽层)同时共挤出。这种制造方法在层之间创建分子粘合界面,确保在电缆弯曲、温度循环或机械应力期间不会发生分层。层之间的粘合如此完整,以至于绝缘屏蔽层无法干净地剥离,这将粘合屏蔽层与某些其他国际标准中使用的可剥离屏蔽层设计区分开来。

AS/NZS 1429.1 mandates comprehensive testing of semi-conductive screens to verify their performance characteristics. Partial discharge testing is performed on completed cables at 1.73 times the rated voltage to ensure that no discharge activity exceeding 10 picocoulombs occurs within the cable construction. This test validates that the semi-conductive screens have successfully eliminated all air voids and created the uniform field distribution necessary for long-term reliable operation.¹⁵

AS/NZS 1429.1 要求对半导电屏蔽层进行全面测试以验证其性能特性。在额定电压的 1.73 倍下对完成的电缆进行局部放电测试,以确保电缆结构内不会发生超过 10 皮库仑的放电活动。此测试验证半导电屏蔽层已成功消除所有空气空隙并创建长期可靠运行所需的均匀场分布。

Additional tests verify the bonding strength between screen layers and insulation, the smoothness of extruded surfaces, and the electrical resistivity of the semi-conductive compounds. These quality control measures ensure that cables manufactured to Australian Standards will provide the expected service life of forty years or more in typical distribution applications. The testing regime reflects the critical importance of semi-conductive screens in preventing premature cable failure through partial discharge and electric stress-related degradation mechanisms.¹⁶

其他测试验证屏蔽层和绝缘之间的粘合强度、挤出表面的光滑度以及半导电化合物的电阻率。这些质量控制措施确保按澳大利亚标准制造的电缆在典型配电应用中将提供预期的四十年或更长的使用寿命。测试制度反映了半导电屏蔽层在防止通过局部放电和电应力相关劣化机制导致的电缆过早失效方面的关键重要性。

The Australian requirements for semi-conductive screens align closely with international standards including IEC 60502-2 for medium voltage power cables. While some international standards permit strippable insulation screens that can be mechanically removed during termination, AS/NZS 1429.1 specifies bonded screens that provide superior long-term reliability. This specification choice reflects Australian operating conditions including high ambient temperatures, UV exposure, and challenging soil conditions that demand maximum cable performance and longevity.¹⁷

澳大利亚对半导电屏蔽层的要求与国际标准密切一致,包括 IEC 60502-2 中压电力电缆标准。虽然一些国际标准允许在终端期间可以机械去除的可剥离绝缘屏蔽层,但 AS/NZS 1429.1 规定的粘合屏蔽层提供更优越的长期可靠性。此规格选择反映了澳大利亚的运行条件,包括高环境温度、紫外线暴露和具有挑战性的土壤条件,这些条件要求最大的电缆性能和寿命。

The mandate for semi-conductive screens above 1.1kV represents decades of accumulated engineering experience and field performance data. Early attempts to operate medium voltage cables without proper screening resulted in high failure rates from partial discharge and tracking phenomena. Modern screening requirements have reduced these failure modes to negligible levels, enabling utilities to achieve cable service lives exceeding forty years in distribution networks. This reliability improvement directly translates to reduced maintenance costs, improved power system availability, and enhanced electrical safety for both utility workers and the general public.¹⁸

1.1kV 以上半导电屏蔽层的要求代表了数十年积累的工程经验和现场性能数据。早期尝试在没有适当屏蔽的情况下运行中压电缆导致局部放电和跟踪现象的高故障率。现代屏蔽要求已将这些故障模式降低到可忽略的水平,使公用事业公司能够在配电网络中实现超过四十年的电缆使用寿命。这种可靠性改进直接转化为降低维护成本、提高电力系统可用性以及增强公用事业工作人员和公众的电气安全。

The requirement for semi-conductive elastomer screens in cables rated above 1.1kV under Australian Standards represents a fundamental recognition of the physics governing electric field behaviour in medium voltage insulation systems. These screens serve multiple critical functions including creating uniform radial electric fields, eliminating partial discharge initiation sites, managing tangential stress at cable terminations, and containing the dielectric field within designed boundaries. The 1.1kV threshold marks the transition point where these protective measures become essential rather than optional, as the electric stress levels in higher voltage cables cannot be safely managed through insulation alone.

澳大利亚标准中对额定电压高于 1.1kV 的电缆的半导电弹性体屏蔽层要求代表了对中压绝缘系统中电场行为物理规律的基本认识。这些屏蔽层具有多种关键功能,包括创建均匀的径向电场、消除局部放电引发点、管理电缆终端的切向应力以及将介电场控制在设计边界内。1.1kV 阈值标志着这些保护措施从可选变为必要的过渡点,因为更高电压电缆中的电应力水平无法仅通过绝缘安全管理。

Through precise material specifications, rigorous manufacturing standards, and comprehensive testing requirements, AS/NZS 1429.1 ensures that medium voltage cables deliver the reliability and longevity demanded by Australia’s electrical infrastructure. The semi-conductive screens specified in these standards represent not merely an additional construction layer, but rather an essential technology that enables safe, efficient power distribution at medium voltage levels throughout industrial, commercial, and utility applications.

通过精确的材料规格、严格的制造标准和全面的测试要求,AS/NZS 1429.1 确保中压电缆提供澳大利亚电气基础设施所需的可靠性和寿命。这些标准中规定的半导电屏蔽层不仅仅代表额外的结构层,而是一种基本技术,使工业、商业和公用事业应用中的中压级安全、高效的配电成为可能。