Otis Singleton
Electric stress, also known as electric field stress, is a phenomenon that occurs in high-voltage systems when insulating materials are subjected to high voltages, causing them to suddenly become conductive. This can lead to insulation failure, as the voltage escapes the insulation and seeks ground or earth, creating hotspots and, eventually, cable failures. The electric field stress is numerically equal to the voltage gradient and is influenced by factors such as the sharpness of electrodes, voltage waveform, rate of voltage increase, and moisture content. In high-voltage apparatus, conductors carry the current while insulators prevent the flow of currents in undesired paths. Various techniques and materials, such as functionally graded materials (FGMs), are used to control and mitigate electrical stress in these systems.
Characteristics and associated values of electrical stress:
| Characteristics | Values |
|---|---|
| Electrical stress | Without an insulation shield, voltage leaves the conductor, travelling through the insulation, creating hotspots and cable failures. |
| Insulation shield | Allows stress/flux to travel efficiently and evenly along the cable, dissipating the stress evenly. |
| Cable splicing | Used to contain and control the deflection of flux lines. |
| Cable termination | Control and dissipate as much electrical stress as possible. |
| Lifetime of electrically stressed material | Depends on the magnitude of the electric stress applied and the length of time it has been subjected to such stress. |
| Electric field stress | In modern times, high voltages are used for a wide variety of applications covering power systems, industry, and research. |
| Electric strength | The standard method for testing oils for electric strength is given in BS 148:1998. |
| Electrical overstress (EOS) | A failure state wherein the device is subjected to undesirable voltage, current, or power. |
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What You'll Learn
Electric field stress
The dielectric strength of insulating materials is a key factor in high-voltage applications. It refers to the voltage at which the current starts to increase uncontrollably unless restrained by the external impedance of the circuit. This dielectric strength is influenced by various parameters, including pressure, temperature, humidity, field configurations, the nature of the applied voltage, imperfections in dielectric materials, and the material and surface conditions of electrodes.
The failure of insulation can occur due to the presence of discharges within voids in the insulation or on its surface. Understanding the mechanisms of insulation failure is essential for preventing issues. Electrical stress can lead to insulation failure by causing voltage to escape erratically, creating hotspots, and eventually resulting in cable failures. This phenomenon is observed when an insulation shield, which normally allows stress to travel efficiently and evenly along the cable, is absent or abruptly terminated.
The lifetime of an electrically stressed material is influenced by the magnitude of the stress and the duration of exposure. Electric strength tests are typically conducted at elevated temperatures, as the strength of most materials decreases with increasing temperature. Other factors affecting electric strength include electrode edge sharpness, voltage waveform, rate of voltage increase, moisture content, and specimen thickness.
To manage Electric Field Stress, various techniques and materials are employed. For instance, functionally graded materials (FGMs) are used to decrease the electric field in HV insulators. FGMs contain micro-sized fillers in a base polymer matrix, resulting in a gradation in permittivity. There are two primary types of FGMs: resistive and capacitive grading materials, which help control the distribution of the electric field and heat along the insulator profile.
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Insulation shields
Electrical stress is a phenomenon that occurs when voltage travels erratically through insulation, creating hotspots and eventually leading to cable failure. This can be mitigated through the use of insulation shields, which allow the electrical stress to travel efficiently and evenly along the cable, dissipating the stress and preventing damage.
Power cables, on the other hand, utilise shields to protect users from shock hazards and enhance cable reliability. They achieve this by preventing partial discharges, also known as "corona", through uniform distribution of electrical stress throughout the insulation. This uniform distribution extends the life of the cable by eliminating these partial discharges.
The most common type of shield is the conductive shield, which is made from materials such as carbon black-loaded polymer or carbon black-impregnated fabric tape. These materials exhibit electrical conductivity between that of a metallic conductor like copper and an insulation material. Conductive shields must be smooth, cylindrical, and free from defects to avoid electrical stress concentrations that could result in insulation damage.
Additionally, power cables rated 5 to 35 kV or higher often incorporate copper tape shields. These copper tapes are applied helically over a semiconducting polymer insulation shield. Copper tape shields increase insulation life by maintaining uniform electrical stress and provide low end-to-end resistance for the shield system. In some cases, copper wire shields are used in combination with copper tape to further enhance the shield fault current capacity.
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Electrical overstress (EOS)
The Industry Council on ESD Target Levels published a white paper in 2016 that defines EOS as follows: "An electrical device suffers an electrical overstress event when a maximum limit for either the voltage across, the current through, or power dissipated in the device is exceeded and causes immediate damage or malfunction, or latent damage resulting in an unpredictable reduction of its lifetime."
EOS can occur due to various factors, such as incorrect power sources, orientation, or voltage surges. For example, applying voltage above the specified limit to a particular device can cause EOS damage. In some cases, handling printed circuit boards (PCBs) without proper Electrostatic Discharge (ESD) precautions can also lead to electrical degradation and EOS. It is important to distinguish between EOS and ESD; while ESD refers to the rapid transfer of electrostatic charge between two objects at different charge levels, EOS describes extreme signals and a wide range of outcomes resulting from multiple stresses or root causes.
The impact of EOS can vary, with sensitive devices often bearing the brunt of the damage. In some cases, the damage may be irreparable, affecting the layers of the PCB. To prevent EOS, it is crucial to adhere to proper ESD measures, utilize ESD stations and equipment, and ensure that all power systems are correctly set up before connecting to electronic devices. Additionally, data cables should be connected prior to powering up the device, and pin matching should be ensured during cable connections.
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Cable splicing and terminating
Electrical stress refers to the erratic movement of voltage through insulation when there is no insulation shield, creating hotspots and eventually leading to cable failure. The insulation shield allows stress/flux to travel efficiently and evenly along the cable, distributing the stress. When the shield is removed for cable splicing and terminating, different percentages of voltage escape the insulation, creating an area of extreme stress.
The process of cable splicing involves joining two or more cables together to create a continuous electrical connection. On the other hand, cable terminating involves preparing the end of a cable to be connected to a device or component. Both of these processes are critical in electrical systems and require careful consideration of electrical stress.
When performing cable splicing, the goal is to control the electrical stress rather than eliminate it. This can be achieved through various methods, including the use of gradient mastic and "heat shrink stress control tubes" in cable splice kits. These tubes help contain and control the deflection of flux lines. Alternatively, hand tape kits utilise different types of tape applied in a specific order and according to precise measurements to manage electrical stress.
In cable terminating, the focus shifts from controlling and containing electrical stress to maximally controlling and dissipating it. Similar techniques can be employed, such as mastic and heat shrink stress control tubing, a solid push-on mass, or hand taping. Regardless of the specific method chosen, the primary objective is to minimise electrical stress and its detrimental effects on the cable's integrity.
It is essential to engage qualified professionals for cable splicing and terminating work. As highlighted by Doug, an expert in the field, attention to the quality of work is paramount. Semi-con removal and cable sanding are often overlooked but crucial steps in ensuring the effectiveness of the electrical stress control measures employed.
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Electric stress on materials
The electric stress on a material can lead to intense heat and eventual failure. This is due to the voltage leaving the conductor and travelling through the insulation, creating "hot spots" and cable failures. This can be mitigated through the use of an "insulation shield", which allows the stress to travel evenly along the cable. However, when the insulation shield is removed for cable splicing and terminating, an abrupt end to the shield allows voltage to escape at a concentrated point, creating an area of extreme stress.
To control and dissipate electrical stress, various methods such as gradient mastic, heat shrink stress control tubes, and hand taping are used. The goal is to achieve safe operating stress through experience, safety factors, and comparative test data. The lifetime of an electrically stressed material depends on the magnitude of the stress applied and the length of exposure.
In addition, the breakdown voltage (BDV) of a material can be affected by the shape and dimension of electrodes, with spherical-pyramidally capped point electrodes exhibiting higher BDV than spherical-cylindrical ones. Materials such as polyphenylene sulfide exhibit higher thermal stability, while polyamide materials experience a significant decrease in BDV due to moisture absorption.
Furthermore, functionally graded materials (FGMs) are used to decrease the electric field on high-voltage (HV) insulators. FGMs contain micro-sized fillers incorporated into a polymer matrix, resulting in a gradation in permittivity. There are two types of FGMs: resistive and capacitive grading materials. These materials can improve the distribution of the electric field and heat along the insulator profile.
Recent advancements have also led to the creation of a new organic material that generates electricity when subjected to mechanical stress. This flexible, rubbery material, developed by researchers at Empa, utilizes the piezoelectric effect to convert mechanical movement into electrical energy, offering potential applications in artificial skin sensors and pacemakers.
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Frequently asked questions
Electrical stress refers to the erratic movement of voltage through insulation when there is no insulation shield, creating hotspots and eventually leading to cable failure.
Electrostatic discharge (ESD) is an important electrical stress that has been widely discussed in technical literature, standards bodies, and educational resources.
The main types of electrical stress can be categorized into over-voltage, over-current, or over-power.
Electrical stress can cause immediate damage or malfunction to a device, or latent damage that results in an unpredictable reduction of its lifetime.
