Wiktor Wise
Electrical stress refers to the erratic movement of voltage through insulation without an insulation shield. This movement creates hot spots and can eventually lead to cable failure. The insulation shield allows stress or flux to travel efficiently and evenly along the cable, dissipating the stress. Electrical stress is a concern in high-voltage applications, where it can lead to electrical breakdown or dielectric breakdown. This occurs when an insulating material is subjected to a high voltage and suddenly becomes a conductor, allowing the flow of current. The lifetime of an electrically stressed material depends on the magnitude of the stress and the duration of exposure.
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What You'll Learn
Electric field stress
In high-voltage applications, the Electric Field Stress on insulating materials is numerically equal to the voltage gradient. This stress can cause erratic voltage behaviour, leading to equipment failure and safety hazards. Therefore, it is essential to carefully design insulation and electrostatic field profiles to prevent undesirable outcomes.
The dielectric strength of an insulating material is the maximum dielectric stress it can withstand before electrical breakdown occurs. Electrical breakdown happens when the insulating material, subjected to a high enough voltage, suddenly becomes a conductor, allowing current flow. The specific breakdown mechanisms differ for solids, liquids, or gases, and even theoretically in a vacuum.
The electric breakdown strength of insulating materials depends on various parameters, including pressure, temperature, humidity, field configurations, voltage nature, imperfections in dielectric materials, and electrode characteristics. The most common cause of insulation failure is the presence of discharges within voids or on the insulation surface.
Additionally, the lifetime of electrically stressed materials is influenced by the magnitude of the electric stress applied and the duration of exposure. This highlights the importance of effective stress control techniques, such as mastic and heat shrink stress control tubing or functionally graded materials, to ensure the safe and reliable operation of high-voltage equipment.
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Insulation shields
Electrical stress refers to the electric field strength or voltage gradient in a given medium, and it is a critical consideration in the design and operation of electrical equipment. When a voltage is applied across an insulator, an electric field is established within it, and this field exerts a force on the electrons in the atoms of the insulating material. This force tends to push the electrons, creating a potential difference and causing a flow of charge, or leakage current, through the insulator. The presence and magnitude of this leakage current depend on the electric field strength and the ability of the insulating material to resist electrical breakdown. This electric field and the resulting stress it places on the insulating material are known as electrical stress.
To mitigate the effects of electrical stress and prevent electrical breakdown, insulation shields are used. Insulation shields are a critical component in electrical systems, especially in high-voltage applications, as they provide a physical barrier between energized parts and surrounding conductive materials or surfaces. Their primary function is to control and distribute the electric field, reducing the risk of arcing, flashovers, and electrical discharges that could damage equipment and lead to system failures.
- Epoxy or Fiberglass: These materials are often used for insulation shields due to their excellent dielectric properties, mechanical strength, and ability to withstand high temperatures.
- Polymeric Materials: Polyethylene, polypropylene, and polytetrafluether, for example, offer flexibility, ease of installation, and resistance to environmental factors.
- Ceramic and Composite Materials: These provide superior arc resistance and thermal stability but may be more brittle and challenging to work with.
The specific material chosen depends on the application's voltage level, environmental conditions, and physical requirements, such as space constraints or flexibility needs.
The design of insulation shields involves careful consideration of the electric field distribution. The shape and placement of the shields are critical to ensuring a uniform electric field and minimizing field concentrations that could lead to breakdown. Conical, cylindrical, or tapered shapes are often used to guide and control the electric field, with the shield's thickness and length depending on the voltage and the insulating material's characteristics.
In addition to field control, insulation shields also provide a physical barrier, preventing accidental contact with energized parts and offering mechanical protection. They are particularly important in compact electrical designs, where the proximity of conductive components or surfaces increases the risk of arcing and flashovers. By using insulation shields, the electrical stress is managed, and the reliability and safety of the equipment are significantly enhanced.
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Cable terminations
Electrical stress refers to the erratic movement of voltage through insulation when there is no insulation shield. This movement creates hotspots and can lead to cable failure. The creation of an insulation shield allows for the even distribution of voltage along the cable, reducing electrical stress.
To address this challenge, various techniques can be employed:
- Mastic and heat shrink stress control tubing: This method involves using a heat-shrink tube that contains gradient mastic to control and dissipate electrical stress through deflection.
- Solid push-on mass: A solid mass is pushed onto the termination to help dissipate electrical stress.
- Hand taping: Specific types of tape are applied in a precise order and according to specific measurements to manage electrical stress effectively.
By utilising these techniques, cable terminations can effectively control and dissipate electrical stress, improving performance and extending the life expectancy of the cable.
Thorne & Derrick International, for instance, are specialist distributors of LV, MV & HV Cable Installation, offering a range of products and services to manage cable terminations and electrical stress, including duct seals, cable cleats, and cable glands.
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Electrical aging
Electrical stress refers to the erratic movement of voltage through insulation, creating hotspots and eventual cable failure. Electrical aging is a concern for the industry, as it depends on the magnitude of electrical stress applied to a material and the duration of exposure.
The lifetime of a material is primarily determined by electrical contacts that control carrier injection and the type and concentration of carrier traps that influence the degradation process. Electrical aging is influenced by the material's chemical composition, source, heat resistance, and physical state. Degradation in liquids and gases occurs due to contamination, solid impurities, and dissolved gases. In solids, electrical breakdown leads to insulation failure, but electrical stress is not the primary aging factor under normal conditions. Instead, thermal and mechanical stresses are more significant contributors to degradation.
The presence of electrical stress can be detected during tests, but aging effects caused by certain aspects of electrical stress, such as the tree phenomenon or insulation surface breaks, may not be identified through simple, non-destructive measurements. Electrical aging is a complex process influenced by various factors, and there are currently no well-defined accelerated aging test methods to determine the lifetime of electrical insulation materials.
To study electrical aging, accelerated aging tests are employed, such as the accelerated water-treeing test (AWTT) and the accelerated cable-life test (ACLT). These tests are designed to estimate the lifespan of assets within a shorter timeframe. Additionally, electric pulse current (EPC) has been introduced to the stress relaxation aging process of certain alloys, resulting in enhanced mechanical properties and delayed aging.
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High-voltage apparatus
Electrical stress refers to the erratic movement of voltage through insulation when there is no insulation shield. This creates hotspots and can lead to cable failure.
In high-voltage apparatus, the most important materials are conductors and insulators. Conductors carry the current, while insulators prevent the flow of currents in undesired paths. The dielectric strength of an insulating material refers to the maximum dielectric stress it can withstand. This strength is dependent on several factors, including pressure, temperature, humidity, field configurations, and the nature of the applied voltage.
The performance of semiconductive stress control materials in high-voltage cable terminations can be improved by using sheds. However, sheds are costly and require additional space, so alternative methods that do not rely on sheds are desirable.
High-voltage cable terminations can be controlled by electrical stress control means, such as stress cones, tapes, or tubular articles made of semiconductive stress control material. These methods aim to control and dissipate electrical stress through deflection.
Thorne & Derrick International, for example, specializes in distributing LV, MV & HV Cable Installation, which includes high-voltage cable accessories from manufacturers like Raychem.
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Frequently asked questions
Electrical stress refers to the erratic movement of voltage through insulation without an insulation shield, as it seeks ground/earth, creating hotspots and eventual cable failure.
Electrical stress is caused by the removal of the insulation shield during cable splicing and terminating. The abrupt end to the shield allows voltage to escape the insulation at a concentrated point, creating an area of extreme stress.
Electrical stress can cause intense heat, leading to cable failure and potential fires. It can also result in the breakdown of insulating materials, causing electrical equipment to fail.
Electrical stress can be controlled through various methods such as mastic and heat shrink stress control tubing, solid push-on mass, or hand taping. The goal is to dissipate and deflect the electrical stress as much as possible.
Conductors and insulators are essential materials used in high-voltage apparatus to manage electrical stress. Conductors carry the current, while insulators prevent the flow of currents in undesired paths.
