Cables are classified depending upon the material used for insulation such as paper, rubber or asbestos. Paper tapes of about 10 cms to 15 cms thickness can be wound on to a conductor in successive layers to achieve a required operating voltage and is used for voltages of 10 kV and above. In the mass-impregnation construction the paper is lapped on in its natural state and is then thoroughly dried by the combined application of heat and vacuum. It is then impregnated with insulating compound. The cable is heated in a hermetically sealed steam-heated vessel to a temperature of 120°-130°C before vacuum is applied. The compound to be used for impregnation is heated to almost 120°C in a separate vessel and is then admitted in the cable vessel. The compound fills all the pores in the paper nation and all the spaces in the cable assembly. After impregnation the cable is allowed to cool down in the compound in order to minimize void formation due to shrinkage. The metal sheath is then applied.
In case of pre-impregnated construction, the papers are dried and impregnated ore application to the conductor and after that there is no drying or impregnation process. The cables are further subdivided into solid, oil-filled or gas-filled types depending upon how the paper insulation impregnated.
For mass impregnated cable when they are laid on a gradient, the compound used for impregnation tends to migrate from the higher to lower level. Thus, voids are formed in the cable at the higher level and because of higher pressure of oil in the lower level cable, the compound will try to leak out. For voltages more than 10 kv, it is the void formation which has been responsible for breakdown.
Three-phase solid paper insulation cables are of two types: (i) the belted type and, (ii) shielded type. The belted type consists of three separately insulated conductors with an overall insulating tape enclosing all the three conductors and finally the metallic sheath is applied. The major disadvantage of belted type construction is that the electric stress is not purely radial. The existence of tangential stresses forces a leakage current (not the charging current) to flow along the layers of paper and the loss of power sets up local heating. It is to be noted that the resistance and dielectric strength of laminated paper is much less along the layers as compared to that across the layers. The local heating of the dielectric may result in breakdown of the material. The breakdown phenomenon due to tangential electric stress is shown in Fig.1.
The tangential stresses are eliminated in case of the shielded construction. In this each conductor is individually insulated and covered with a thin metallic non-magnetic shielding tape. The three shields are in contact with each other and the three conductors behave as three singe-phase conductors. The three conductors are then cabled together with an additional shield wrapped round them. There is no belt insulation provided but it is lead covered and armored. All the four shields and the lead sheath are at earth potential and, therefore, the electric stresses are radial only; thereby, the tangential stresses are completely eliminated. the 3-phase shielded construction cable is shown in Fig.2.
The following are the method for elimination of void formation in the cable:
- The use of low velocity mineral oil for impregnation of the dielectric and the inclusion of oil channels so that any tendency of void formation (due to cyclic heating and cooling of impregnant) is eliminated.
- The use of inert gas at high pressure within the metal sheath and in direct contact with the dielectric.
The first method is used in oil-filled cables. Oil ducts are provided within the cable itself and they communicate with oil tanks provided at suitable locations along the cable route so as to accommodate any changes in the oil volume during heating and cooling process fig.3.
Single phase oil cables consist of a concentric stranded conductor built around an open helical spring core which serves as a channel for the flow of oil. The cable is insulated and sheathed in the same manner as the solid type cables. The 3-phase cables are normally of the shielded design type and consist of three oil channels composed of helical springs that extend through the cable in spaces normally occupied by filler material Fig.4. Another, design of three-core oil filled cable is the flat type as shown in Fig.5, The flat sides are reinforced with metallic tapes and binding wires so that during increase in pressure of oil, due to heating, the flat side is deformed and the section of the cable becomes slightly elliptical. Yet another construction of 3-core oil filled cables uses 3-core paper insulated cable without a lead sheath. The cable is pulled into a steel pipe which then is filled with oil. Pumps are then used to maintain a specified oil pressure and allow it to expand and contract with the loading cycle.
Leakage or oil in these cables is a very serious problem. Automatic signaling is, therefore, installed to indicate the fall in oil pressure in any of the phases. Oil filled cables require relatively smaller amount of insulation as compared to solid type for the same operating voltage and are recommended for all voltages ranging between 66 kV and 400 kv.
To obviate the disadvantages of oil filled cables in terms of expansion and contraction of oil during loading cycles, the gas filled cables are used which have a self-contained compensating arrangement within the confines of the lead sheath. The compression cable is fundamentally a solid type construction with two important modifications; (i) the cable cross section is non-circular and (ii) the sheath thickness is reduced to allow the cable to breathe more easily. The cable is then surrounded with an envelope and the space between the two is filled with an inert gas at a nominal pressure of 14 kg/cm2 which compresses the cable dielectric via the diaphragm sheath. During heating, the cable compound expands and travels radially through the dielectric and a space is provided by it by movement of the sheath, the non-circular shape becomes circular there. When the cable cools down, the gas pressure acting via the metallic sheath, forces the compound back into the paper insulation.
The gas cushion cable consists of stranded conductor, paper insulated, screened, lead sheathed, metallic reinforced and with a rubber-containing water proof covering. A continuous gas space throughout the length the cable is provided. The inert gas introduced is at high pressure within the lead sheath and in contact with the dielectric in order to suppress gaseous ionization.
The impregnated pressure cable is similar to solid type except that provision is made for longitudinal gas flow, the cable has a mass-impregnated insulation design and is maintained under a gas pressure of 14 kg/cm2. ln single core cables the sheath clearance is about 0.175 cm, and in 3-core cables about 0.075 cm. In case of 3-core cables, a lead gas channel pipe is provided which is located in the space normally occupied by the filler Fig.6. The object of this pipe is to provide low resistance path between joints.
Because of the good thermal characteristic and high dielectric strength of the gas SF6, it is used for insulating the cables also. SFs gas insulated cables can be matched to overhead lines and can be operated corresponding to their surge impedance loading. These cables can be used for transporting thousands of MVA even at UHV whereas the conventional cables are limited to 1000 MVA and 500 kV.
Also Read- Types of Insulators