Trystan Casey
Electrical stress refers to the voltage that leaves a conductor, travelling through insulation and seeking ground/earth. This movement creates hot spots and eventually leads to cable failure. Electrical stress can be controlled and dissipated through various methods, such as mastic and heat shrink control tubing, push-on mass, or hand taping. The goal is to manage the stress and prevent it from causing damage to the cable or insulation. In some cases, an insulation shield is used to allow the stress to travel efficiently and evenly along the cable, distributing the electrical stress.
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What You'll Learn
Voltage escaping insulation
Electrical stress can be controlled through deflection using mastic and heat-shrink control tubing, a solid push-on mass, or hand taping. However, the type of cable termination is irrelevant as they all aim to achieve the same result.
The creation of an "insulation shield" allows the stress or flux to travel efficiently and evenly along the cable, evenly dissipating the stress. The problem arises when the insulation shield is removed for cable splicing.
To prevent voltage from escaping insulation, it is crucial to test the integrity of the insulation by measuring its resistance to current flow. A high level of resistance indicates that the insulation is effectively containing the current, while a low level of resistance suggests that a significant amount of current may be leaking through. By pressurizing a conductor with a given voltage, Ohm's Law (R=V/I) can be used to quantify resistance measurements.
Additionally, functional insulation, also known as operational insulation, is a type of insulation that ensures the correct function or operation of a product. It is not intended to isolate the user from dangerous voltages. An example of functional insulation is the enamel insulation around the wire used to wind a coil, which prevents adjacent windings from shorting together.
Supplementary insulation is another important concept. It serves as a second layer of insulation independent of the basic insulation. Supplementary insulation provides an additional safeguard against hazardous voltages in the event of a failure in the primary insulation.
In conclusion, voltage escaping insulation is a critical aspect of electrical stress management. By employing insulation shields, testing insulation integrity, and utilizing supplementary insulation layers, we can effectively control and mitigate the negative consequences of voltage escaping through insulation.
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Insulation shields
Electrical stress is a phenomenon that occurs when voltage escapes the insulation and travels erratically, seeking ground or earth. This creates hotspots and eventually leads to cable failure. Insulation shields are a crucial component in managing electrical stress.
An insulation shield is a protective barrier that helps control and evenly dissipate electrical stress in cables. It allows the stress or flux to travel efficiently and uniformly along the cable, preventing the concentration of voltage at a single point. The shield is typically made of conductive or semiconductive materials, such as carbon black-loaded polymer, carbon black-impregnated fabric tape, or copper tape. These materials have electrical conductivity levels between metallic conductors like copper and insulation materials.
One of the most popular types of insulation shields is the conductive shield. It is constructed of materials with electrical conductivity, such as an extruded carbon black-loaded polymer or carbon black-impregnated fabric tape. Conductive shields are commonly referred to as ""semiconductive" shields. It is important to maintain the smoothness, cylindrical shape, and cleanliness of semiconductive shields to prevent electrical stress concentrations that can damage the insulation. These shields play a crucial role in protecting the insulation from the detrimental effects of corona while ensuring intimate contact with the insulation, free from voids and defects.
Power cable shields are another type of insulation shield that helps protect users from shock hazards and enhances cable reliability. They achieve this by preventing partial discharges (corona) within the cables. Power cable shields are typically metallic coverings that enclose insulated conductors or groups of conductors. By utilising these shields, the electrical shock hazard to individuals is reduced, and a uniform distribution of electrical stress is achieved throughout the insulation. This uniform distribution prolongs the life of the cable by eliminating partial discharges.
Additionally, copper tape is commonly used in power cable shields, especially for cables rated 5 to 35 kV and above. The copper tape is applied helically over a semiconducting polymer insulation shield. In combination with the insulation shield, copper tape helps maintain uniform electrical stress and provides low end-to-end resistance within the shield system.
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Electric field control
Electrical stress is a phenomenon that occurs when voltage escapes an insulation shield and travels through the insulation, seeking ground or earth. This creates hotspots and can lead to cable failure. Electric field control methods are essential to mitigate these issues and ensure the reliable operation of equipment.
One approach to electric field control is the use of functionally graded materials (FGMs). FGMs are smart materials that respond to electrical stimuli and can be used to decrease the electric field in high-voltage applications. They are effective in gas-insulated switchgears (GIS), bushings, cable terminations, and winding insulation. The composition of FGMs can be varied to achieve resistive or capacitive grading, improving the distribution of the electric field and heat along the insulator profile.
Another technique for electric field control is the use of stress cones, also known as geometric stress control. This method involves positioning a conductive cone-shaped component at the screen edge of MV joints and terminations, forcing the equipotential lines to separate and exit the cable insulation more gradually. This reduces the electric stress at the screen edge and other high-stress areas.
Additionally, electric field control can be achieved through the application of materials with special electrical impedance characteristics. These materials, similar to surge arresters, have high relative permittivity, which grades the electric field along their length and reduces field strength at critical points. Such materials can take the form of heat-shrink or cold-shrink tubings, mastics, or hot-melt compounds.
It is important to note that effective stress control requires not only the use of appropriate components but also their correct installation. Installers must ensure that accessory insulation is void-free, especially in high-stress areas. This includes properly positioning cold-shrink and push-on components, applying uniform heating to heat-shrink components, and carefully following installation instructions for stress control tapes.
In summary, electric field control methods are crucial in managing electrical stress and maintaining the reliable operation of equipment. By employing techniques such as FGMs, stress cones, and materials with special electrical impedance characteristics, along with careful installation practices, it is possible to reduce electrical stress and extend the lifespan of electrical systems.
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Cable splicing
Electrical stress occurs when voltage escapes the insulation at a concentrated point, creating an area of extreme stress as it seeks ground/earth. This can cause intense heat and eventually cable failure. In the context of cable splicing, the removal of the insulation shield can lead to electrical stress. The goal is not to eliminate this stress but to control and contain it.
There are several methods to control electrical stress in cable splicing. One approach is to use gradient mastic and "heat shrink stress control tubes" to contain and control the deflection of flux lines. These tubes are designed to manage the stress by providing a controlled path for the voltage to follow, preventing it from escaping the insulation erratically.
Another method is to use push-on single-mass control tubes. These tubes are solid and provide a physical barrier between the spliced cables, guiding the electrical stress away from the joint. Additionally, hand taping is a technique where different types of tape are applied in a specific order and according to precise measurements to control the stress at the splice point.
It is important to note that the quality of work in cable splicing is of utmost importance. Semi-con removal and proper cable preparation, such as sanding, are critical steps to ensure effective stress control. Specialists in cable installations, such as Thorne & Derrick International, offer a range of cable splicing products and services to manage electrical stress and ensure safe and reliable cable joints.
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Electrical strength tests
Electrical stress is the result of voltage escaping insulation and seeking ground or earth, creating hotspots and eventually leading to cable failure. Electrical stress testing, also known as dielectric strength testing, is a critical step in ensuring the safety of electrical and electronic devices. The purpose of these tests is to determine the highest stress that a material or assembly can withstand indefinitely. This helps to ensure that the insulation is adequate and not marginal, preventing electrical failures and potential safety hazards.
One commonly used electrical strength test is the HIPOT test, which is a nondestructive test that assesses the effectiveness of electrical insulation in devices. This test is conducted by applying high voltage to a device for a specific amount of time, typically up to 5 minutes. During the test, the voltage may be either direct current or alternating current, and it can be adjusted to manage leakage currents. The hipot tester's current trip level should be set high enough to avoid failure due to leakage current. Only qualified workers should perform this type of testing, and safety guidelines must be followed to minimize the risk of electrical shock.
Another important aspect of electrical strength testing is understanding the factors that influence apparent electric strength. These include the sharpness of electrode edges, voltage waveform, the rate of voltage increase, moisture content, specimen thickness, and the medium in which the test is conducted. Comparative values at different temperatures, thicknesses, and test durations are often obtained to make informed decisions. Additionally, tests with lightning or switching-impulse voltages may be conducted for materials that will be subjected to transient voltages in operation, such as overhead-line insulators or switchgear.
Electrical strength testing is a complex process that requires expertise and adherence to safety standards. Manufacturers of electrical products often find the compliance journey challenging due to the multitude of rules and regulations. However, by working with specialized testing laboratories and experts in the field, they can ensure their products meet the required standards and pass safety certifications. These laboratories offer comprehensive testing services, providing high-quality data, test reports, and certificates to support manufacturers in making informed decisions about their materials and compliance obligations.
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Frequently asked questions
Electrical stress is what happens when voltage leaves a conductor and travels through insulation, creating hot spots and eventually leading to cable failure.
Electrical stress is caused by the absence of an insulation shield, which normally allows voltage to travel efficiently and evenly along a cable.
Electrical stress can cause intense heat and eventually cable failure.
Electrical stress can be controlled by using an insulation shield, or by using cable splice kits with gradient mastic and heat shrink stress control tubes.
