Contenido

Aunque los sistemas de alta tensión se definen como aquellos que operan a cualquier voltaje superior a 1000 V, la mayoría de los cables se usan para tensiones superiores a los 10 000 V. Aquellos cables que operan en el rango de 10 a 33 kV son llamados cables de media tensión, y los que trabajan por encima de los 50 kV se denominan cables de alta tensión..

Los cables modernos de alta tensión tienen un diseño simple, consistente de pocas partes. De acuerdo con la figura 1, los cables de alta tensión están compuestos por las siguientes partes:.

• - Conductor: Es el encargado de transportar la corriente, y normalmente es fabricado en cobre o aluminio. Los conductores con sección transversal de 2000 mm² pueden transportar corrientes de hasta 2000 amperios. Generalmente es habitual que a cada fibra individual se le deforme previamente para facilitar una mejor circunferencia.

• - Pantalla semiconductora interna: Su función es distribuir uniformemente el campo eléctrico alrededor del conductor. Está integrada en la capa interna del aislamiento.

• - Aislamiento: Soporta el campo eléctrico en el interior del cable. Está compuesto por un material aislante como el XLPE, el cual es razonablemente flexible y puede operar en temperaturas de hasta 120 °C.

• - Pantalla semiconductora externa: Su función es distribuir uniformemente el campo eléctrico alrededor del aislamiento. Está integrada en la capa externa del aislamiento. La función de ambas pantallas semiconductoras es prevenir cavidades llenas de aire entre el metal conductor y el dieléctrico, evitando pequeñas descargas eléctricas") que podrían dañar el aislamiento.

• - Pantalla metálica:Sirve como conductor a tierra y conducirá la corrientes de pérdida si es necesario. Dependiendo del diseño, está hecha por hilos de cobre que rodean el cable de forma helicoidal, o bien, una lámina de aluminio o cobre.

Quality

During the development of high voltage insulation, which has taken nearly half a century, two characteristics were found to be fundamental to guarantee cable quality. First, the use of semiconductor layers. These layers must be absolutely smooth, without even protrusions of a few micrometers "Micrometer (unit of length)"). In addition to this, the fusion between the insulation and these layers must be absolute; Any fission, air bubble, or any other defect of the same dimensions mentioned above may impair the failure characteristics of the cable.

Another important aspect is that the insulation should not have any inclusion, cavity or any other similar defect. Any defect of this type shortens the life of the cable which should be in the order of 30 years or more.

Cooperation between cable manufacturers and material manufacturers has resulted in XLPE being classified to very precise specifications. Most XLPE manufacturers specify an “extra clean” classification, where the number and size of foreign particles are guaranteed. The raw material is required to be packaged and stored inside a clean room in the machine that manufactures the cable. The development of plastic extruders and cross-linking have made the manufacturing process free of defects and impurities in the insulation. The final quality control is to perform a very high intensity 50 or 60 Hz partial discharge test (in the range of 5 to 10 picocoulombs). This test is carried out on each reel that leaves the factory.

A cable for high voltage direct current transmission has the same construction as an alternating current cable. The physics and testing requirements are different. In this case the softness of the semiconducting layer is of utmost importance. Cleaning the insulation remains important.

Many HVDC cables are used for direct current submarine connections, because distances greater than 30 km cannot be used on alternating current. The longest cable currently installed is the Norned cable connecting Norway and Holland, which is about 600 km long and carries 600 MW, a capacity similar to a high-power substation.

The terminals of high voltage cables must have the ability to control the electric field at the ends. Without the terminal, the electric field would be concentrated at the end of the ground conductor, as seen in Figure 2.

The equipotential lines observed in Figure 2 are similar to the isolines of a mountainous region: the closer these lines are to each other, the steeper the slope and the greater the damage; which in this case is equivalent to a breakdown of the dielectric strength of the material.

To control the equipotential lines (that is, to control the electric field), a device called a stress cone or relief cone is used, as seen in Figure 3. This device widens the screen in a logarithmic curve. Before 1960, relief cones were made manually with tape, after the installation process. These cones were protected with a bushing, which on the outside was an insulator and on the inside was filled with a dielectric compound. After the 1960s, the premolded terminal was developed, which consisted of a rubber or elastomer body that was stretched over the end of the cable. This body R made of rubber has an electrode added to the outer layer, which is responsible for redistributing the equipotential lines to guarantee a low concentration of the electric field.

The objective of this device, which was invented by NKF in 1964, is that the diameter of the perforation of the elastic body is smaller than the diameter of the cable. In this way, the interface between the cable and the relief cone is achieved by mechanical pressure, preventing cavities or bubbles from forming between the cable and the cone. Thus, dielectric strength is prevented in this region of the cable.

Subsequently, in the construction stage, the device can be protected with a porcelain or silicone insulator for outdoor use, or with accessories that allow the cable to be connected to a transformer, or a gas-insulated substation.

Joining two high-voltage cables causes two problems: First, the outer semiconductor layer must be coupled so that there is no field concentration, as is the case at the cable terminals. Second, a field-free space must be generated where the insulation ends and where the connector of both conductors is located. These problems were solved by NKF in Delft in 1965.

Figure 4 shows the cross section of a splice. On one side of the photograph you can see the contour lines of a high voltage cable. Here red represents the conductor of the cable and blue represents the insulation of the cable. The black parts of the image correspond to the semiconductor parts of the junction. The external part is connected to ground and distributes the electric field in the same way as a cable terminal. The internal part is at high voltage and is responsible for shielding the connector from the electric field.

The field is redistributed so that the equipotential lines are directed smoothly from the inside of the cable to the outside of the splice.

It should be noted that as in the terminal, the diameter of the device is smaller than the diameter of the cable insulation. In this way, a permanent pressure is created between the device and the surface of the cable in order to avoid cavities or weak electrical points.

Installing a splice is a job carried out by highly specialized personnel. The process of removing the outer semiconducting layer at the ends of the cable, locating the relief cones, joining the conductors, etc., requires skill, cleanliness in the process, and precision.

X-ray cables are used in lengths of several meters to connect the high voltage source to an X-ray tube or any other high voltage device that is part of scientific equipment. These cables transmit small currents on the order of milliamps, with voltages between 30 and 200 kV direct current, or sometimes even higher. The cables are flexible, with rubber insulation or other dielectric material, the core is made up of stranded conductors and a sheath of stranded copper wire. The construction has the same elements as other high voltage cables.

There are different reasons why cable insulation can be defective, given that solid or paper dielectric materials are used as insulation. Therefore, there are several test and measurement methods to verify that a cable is fully functional or to detect faults. While paper cables are primarily tested for insulation resistance with direct current, the most common test for cable with a solid dielectric is to cause a partial discharge. It is necessary to distinguish between validating a cable and diagnosing a cable. While cable validation methods result in a pass/fail statement, cable diagnostic methods allow the current condition of the cable to be evaluated. With some tests, it is even possible to locate the position of the defect in the insulation before failure occurs.

• - Electrical energy transmission.

• - High voltage direct current.

Contenido

Aunque los sistemas de alta tensión se definen como aquellos que operan a cualquier voltaje superior a 1000 V, la mayoría de los cables se usan para tensiones superiores a los 10 000 V. Aquellos cables que operan en el rango de 10 a 33 kV son llamados cables de media tensión, y los que trabajan por encima de los 50 kV se denominan cables de alta tensión..

Los cables modernos de alta tensión tienen un diseño simple, consistente de pocas partes. De acuerdo con la figura 1, los cables de alta tensión están compuestos por las siguientes partes:.

• - Conductor: Es el encargado de transportar la corriente, y normalmente es fabricado en cobre o aluminio. Los conductores con sección transversal de 2000 mm² pueden transportar corrientes de hasta 2000 amperios. Generalmente es habitual que a cada fibra individual se le deforme previamente para facilitar una mejor circunferencia.

• - Pantalla semiconductora interna: Su función es distribuir uniformemente el campo eléctrico alrededor del conductor. Está integrada en la capa interna del aislamiento.

• - Aislamiento: Soporta el campo eléctrico en el interior del cable. Está compuesto por un material aislante como el XLPE, el cual es razonablemente flexible y puede operar en temperaturas de hasta 120 °C.

• - Pantalla semiconductora externa: Su función es distribuir uniformemente el campo eléctrico alrededor del aislamiento. Está integrada en la capa externa del aislamiento. La función de ambas pantallas semiconductoras es prevenir cavidades llenas de aire entre el metal conductor y el dieléctrico, evitando pequeñas descargas eléctricas") que podrían dañar el aislamiento.

• - Pantalla metálica:Sirve como conductor a tierra y conducirá la corrientes de pérdida si es necesario. Dependiendo del diseño, está hecha por hilos de cobre que rodean el cable de forma helicoidal, o bien, una lámina de aluminio o cobre.

Quality

During the development of high voltage insulation, which has taken nearly half a century, two characteristics were found to be fundamental to guarantee cable quality. First, the use of semiconductor layers. These layers must be absolutely smooth, without even protrusions of a few micrometers "Micrometer (unit of length)"). In addition to this, the fusion between the insulation and these layers must be absolute; Any fission, air bubble, or any other defect of the same dimensions mentioned above may impair the failure characteristics of the cable.

Another important aspect is that the insulation should not have any inclusion, cavity or any other similar defect. Any defect of this type shortens the life of the cable which should be in the order of 30 years or more.

Cooperation between cable manufacturers and material manufacturers has resulted in XLPE being classified to very precise specifications. Most XLPE manufacturers specify an “extra clean” classification, where the number and size of foreign particles are guaranteed. The raw material is required to be packaged and stored inside a clean room in the machine that manufactures the cable. The development of plastic extruders and cross-linking have made the manufacturing process free of defects and impurities in the insulation. The final quality control is to perform a very high intensity 50 or 60 Hz partial discharge test (in the range of 5 to 10 picocoulombs). This test is carried out on each reel that leaves the factory.

A cable for high voltage direct current transmission has the same construction as an alternating current cable. The physics and testing requirements are different. In this case the softness of the semiconducting layer is of utmost importance. Cleaning the insulation remains important.

Many HVDC cables are used for direct current submarine connections, because distances greater than 30 km cannot be used on alternating current. The longest cable currently installed is the Norned cable connecting Norway and Holland, which is about 600 km long and carries 600 MW, a capacity similar to a high-power substation.

The terminals of high voltage cables must have the ability to control the electric field at the ends. Without the terminal, the electric field would be concentrated at the end of the ground conductor, as seen in Figure 2.

The equipotential lines observed in Figure 2 are similar to the isolines of a mountainous region: the closer these lines are to each other, the steeper the slope and the greater the damage; which in this case is equivalent to a breakdown of the dielectric strength of the material.

To control the equipotential lines (that is, to control the electric field), a device called a stress cone or relief cone is used, as seen in Figure 3. This device widens the screen in a logarithmic curve. Before 1960, relief cones were made manually with tape, after the installation process. These cones were protected with a bushing, which on the outside was an insulator and on the inside was filled with a dielectric compound. After the 1960s, the premolded terminal was developed, which consisted of a rubber or elastomer body that was stretched over the end of the cable. This body R made of rubber has an electrode added to the outer layer, which is responsible for redistributing the equipotential lines to guarantee a low concentration of the electric field.

The objective of this device, which was invented by NKF in 1964, is that the diameter of the perforation of the elastic body is smaller than the diameter of the cable. In this way, the interface between the cable and the relief cone is achieved by mechanical pressure, preventing cavities or bubbles from forming between the cable and the cone. Thus, dielectric strength is prevented in this region of the cable.

Subsequently, in the construction stage, the device can be protected with a porcelain or silicone insulator for outdoor use, or with accessories that allow the cable to be connected to a transformer, or a gas-insulated substation.

Joining two high-voltage cables causes two problems: First, the outer semiconductor layer must be coupled so that there is no field concentration, as is the case at the cable terminals. Second, a field-free space must be generated where the insulation ends and where the connector of both conductors is located. These problems were solved by NKF in Delft in 1965.

Figure 4 shows the cross section of a splice. On one side of the photograph you can see the contour lines of a high voltage cable. Here red represents the conductor of the cable and blue represents the insulation of the cable. The black parts of the image correspond to the semiconductor parts of the junction. The external part is connected to ground and distributes the electric field in the same way as a cable terminal. The internal part is at high voltage and is responsible for shielding the connector from the electric field.

The field is redistributed so that the equipotential lines are directed smoothly from the inside of the cable to the outside of the splice.

It should be noted that as in the terminal, the diameter of the device is smaller than the diameter of the cable insulation. In this way, a permanent pressure is created between the device and the surface of the cable in order to avoid cavities or weak electrical points.

Installing a splice is a job carried out by highly specialized personnel. The process of removing the outer semiconducting layer at the ends of the cable, locating the relief cones, joining the conductors, etc., requires skill, cleanliness in the process, and precision.

X-ray cables are used in lengths of several meters to connect the high voltage source to an X-ray tube or any other high voltage device that is part of scientific equipment. These cables transmit small currents on the order of milliamps, with voltages between 30 and 200 kV direct current, or sometimes even higher. The cables are flexible, with rubber insulation or other dielectric material, the core is made up of stranded conductors and a sheath of stranded copper wire. The construction has the same elements as other high voltage cables.

There are different reasons why cable insulation can be defective, given that solid or paper dielectric materials are used as insulation. Therefore, there are several test and measurement methods to verify that a cable is fully functional or to detect faults. While paper cables are primarily tested for insulation resistance with direct current, the most common test for cable with a solid dielectric is to cause a partial discharge. It is necessary to distinguish between validating a cable and diagnosing a cable. While cable validation methods result in a pass/fail statement, cable diagnostic methods allow the current condition of the cable to be evaluated. With some tests, it is even possible to locate the position of the defect in the insulation before failure occurs.

• - Electrical energy transmission.

• - High voltage direct current.