Basic theory of cable medium
whether electric energy or signal is transmitted by wires and cables, the purpose is to use it to transmit current to the power receiving equipment or power receiving end. To achieve this, a conductor is required to transmit the required load current. It is also important to keep the current flowing along the conductor without being bypassed. Electrical insulation (dielectric) basically separates the conductor from other paths or interfaces and prevents current from flowing along these possible bypasses. Therefore, it can be said that any conductor transmitting electrical signals or energy is an insulated conductor.
2.2 electric field and voltage
current is the characterization of charge motion. It can be assumed that the current carrying conductor and the return conductor are connected to the load, like two parallel cylindrical charge channels. If the conductor diameter (line charge) is ignored, the power line distribution can be represented by an approximate circular curve family; The center of the curve is on the zero potential line; Each circular power line passes through the center of the cylinder. The equipotential (equipotential) line of each charge is perpendicular to its power line direction. The potential at any position in the electric field is the vector sum of the potential under the action of each charge. Because the center of the circular power line is located on a straight line, and the distance between the straight line and the charge is equal, the equipotential lines of each charge cancel each other on this straight line, making its net potential zero
If the zero potential line is replaced by a conductive plane (such as the ground) and only the conductor above the plane is retained, the position of the power line and equipotential line will not change. However, the part below the zero potential line is simplified to the mirror image of the part above the plane (mirror image method). If the conductor diameter is ignored, the power line and equipotential (equipotential) line can be drawn for the current carrying conductor above the ground. Of course, the insulation (medium) in the above example is air.
2.3 air insulated conductor
air is not a good insulating material. Compared with many other insulating materials, its electric field breakdown strength is low. Due to cost factors, it is a widely used insulating medium in places where space is not limited. With the increase of conductor to ground voltage, the electric field on the conductor surface may exceed the breakdown field strength of air. At this time, the air near the conductor will break down and produce a layer of ionized air around the conductor, which is called corona. It will produce energy loss and interfere with audio, video and other signal transmission. This is not uncommon and usually occurs where there are burrs on the surface of conductors or connectors. In these irregular or protruding parts, the surface electric field intensity will increase locally. In air or other gases, the ionized gas layer around the conductor increases the outer diameter of the equivalent electric field; Under the general conditions of temperature, pressure and humidity, the electric field strength at the edge of the gas layer decreases below the breakdown field strength and is no longer ionized. It can be considered that this ionized gas is an unintentional conductor shield. The continuous supply of fresh air and the maintenance of the above state will prevent the ionization process and will not cause all the air to the ground to ionize. If the field strength level is high enough, an ionization channel may also be formed between the conductor and the earth, but this generally requires a very high voltage source, such as lightning.
2.4 use insulation to save space
space is one of the common constraints, which hinders the use of air as insulation. If air insulation is adopted, it can be imagined how large the insulation space requirements of bare wires in buildings will be. It is necessary to consider replacing the air around the conductor with a better insulating material (dielectric).
due to the proximity and contact with other objects, the insulation thickness of low-voltage cables generally depends on the mechanical performance requirements rather than the electrical performance requirements. The characteristic requirements brought by the surrounding environment, such as sunshine, flame retardant and harsh installation conditions, make it difficult for a single insulating material to meet all relevant requirements. The design structure of more than two layers is widely used in the design of low-voltage cables. Although the outer layer is usually insulated, it may sacrifice a certain degree of insulation performance to obtain other properties, such as high strength, sunlight resistance, flame retardant, chemical resistance, etc. In this case, the outer layer of the cable may act as both insulation and sheath.
2.5 higher voltage
since the equipotential line is perpendicular to the power line, this bending will lead to the difference of the surface potential of the insulating layer. At low voltage, the effect can be ignored. With the increase of voltage, the gradient of potential will reach high enough to generate current flowing through the insulating surface, which is generally called “creepage”. Even if the surface current is very small, high surface impedance will still cause heating and eventually damage the insulating layer. If this situation continues, electric corrosion may develop into serious insulation damage; If the insulation is also in contact with the earth, it will cause a ground fault.
2.6 insulation and shielding
requirements for fault current of cables used in power system; A sufficiently large fault current requires a neutral circuit in the design of metal shielding. Such cables are called underground distribution cables and residential underground distribution cables. It is important to understand the function of metal shielding system, because many serious errors and faults are caused by misunderstanding of its function.
2.7 necessary conductor shielding
the existence of insulating shielding creates another complexity. The grounded insulation shield causes all voltage to be applied to the insulation.
as in the case of air insulated conductor, the problem of exceeding the maximum field strength withstood by the insulating layer should be considered here. Irregular edges of stranded conductors and possible burrs and scratches on the surfaces of stranded and solid conductors make this problem particularly significant.
A semi conducting layer is added outside the conductor to alleviate the irregularity of its surface. This can reduce the possibility of protrusions entering the insulating layer. The bulge entering the insulating layer or semi conducting layer will increase the local field strength (field strength concentration), which may exceed the long-term breakdown strength of the insulation. In the case of extruded insulation, the requirements are particularly strict. Unlike air insulation, (solid) insulation cannot be “updated immediately”. Any damage may develop and cause complete breakdown of the insulation.
2.8 requirements for shielding layer
there are several basic requirements for the shielding layer to reduce the concentration of field strength. The first is bulge, which must be minimized whether caused by material smoothness or manufacturing. This bulge weakens the function of shielding and will cause local field strength concentration. The insulation shielding layer also has a more complex problem, that is, it is expected to be easy to peel off and facilitate the installation of joints and terminals. This of course refers to medium voltage conditions (5 ~ 35kV). At higher voltage, the inconvenience caused by non peelable insulation shield is allowed. For non peelable shielded cables, the interface between insulation and insulation shielding can be smoother without micropores.
2.9 insulation requirements
at medium voltage and higher voltage levels, the insulation and insulation shielding interfaces are strictly specified to be free of impurities. Like protrusions, impurities at the interface will lead to field concentration and increase the possibility of breakdown. Micropores also lead to field concentration. For gas filled micropores, the voltage gradient generated inside can cause “capacitance resistance” discharge. Such discharge will damage the surrounding insulating material, resulting in deterioration or breakdown of insulating performance.
2.10 protective layer
in low pressure applications, protective layers are commonly used to protect the bottom layer from physical damage, sunlight and chemical attack. In medium voltage shielded cables, chemical erosion includes the underlying metal layer and armor layer for shielding. In the design of composite conductor, comprehensive sheath is widely used for the same purpose. For medium and high voltage cables, the application of sheath has run through the whole history of cable design. They are used for the same purpose as in low-voltage cables, but it is more important to protect the underlying metal parts from corrosion. The only exceptions are paper insulated, lead sheathed cables, and early URD / UD designs widely used in industrial electrical. Both “experiments” are based on the cable with lead sheath and continuous copper conductor, and the experimental objects are not affected by significant corrosion. The experimental results show that the failure rate increases in these designs. The function of protective layer is to reduce the entry of liquid. It has been proved that the entry of liquid has a negative impact on many media, so it is specially mentioned here.