Cables and Connectors Multiple choice Questions and Answers

Answer.4. All of the above

Insulating material for a cable

The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. Therefore, the proper choice of insulating material for cables is of considerable importance. In general, the insulating materials used in cables should have the following properties

1. High insulation resistance to avoid leakage current.
2. High dielectric strength to avoid electrical breakdown of the cable.
3. High mechanical strength to withstand the mechanical handling of cables.
4. Nonhygroscopic, that is, it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case, the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath.
5. Noninflammable.
6. Low cost so as to make the underground system a viable proposition.
7. Unaffected by acids and alkalies to avoid any chemical action.

Answer.1. Becomes double

The property of a cable system that permits the conductor to maintain a potential across the insulation is known as capacitance. Its value is dependent on the permittivity (dielectric constant) of the insulation and the diameters of the conductor and the insulation. A cable is a distributed capacitor. Capacitance is important in cable applications since the charging current is proportional to the capacitance as well as to the system voltage and frequency. Since the charging current is also proportional to length, the required current, as well as capacitance, will increase with cable length.

Answer.1. Compressed oil or compressed gas insulated

For 220 kV (and higher) cables, it is necessary to avoid cable ionization by preventing void formation. Voids are formed by cyclic heating and cooling of the impregnant. The void formation can be prevented by the use of a very thin film of oil under pressure. Such pressurized cables are known as oil-filled cables.

In this type, a hollow conductor of soft drawn stranded copper, fed by oil reservoirs placed at intervals along the line, is used. The oil is maintained under pressure by these reservoirs. As the cable heats on load, oil is driven from the cable into the reservoirs and vice versa, hence the formation of voids is prevented. Impregnated paper is used for insulation, and a lead sheath and jute covering are employed to give water proofing. The figure depicts a cross-section of a 220 kV oil-filled cable. Oil-filled cables require a relatively smaller amount of insulation as compared to the solid type for the same operating voltage, and, therefore, are used for higher voltages (66 kV-400 kV). At 500 kV, the oil-filled cable has working stress of 15 kV/mm.

Answer.3. Smaller dielectric losses

The current carrying capacity and the voltage rating of a cable increase significantly when used in dc systems as compared to ac applications. This means that the cable can transmit more power in a dc system. The reasons are explained below.

Current Carrying Capacity

The losses in a cable under dc application are less than those with ac, because:

⇒ There is no charging current continuously flowing through the insulation in the dc application, but the charging current flows through the capacitance when the cable is energized from the ac source. The charging current may increase menacingly in EHV ac systems for a few kilometers of the cable. So, a reduction of copper losses in the conductor on this count in dc cables enables the cable conductor to carry more load current.

⇒ In a dc cable, the only power loss in the dielectric will be the copper loss due to leakage current as the dielectric hysteresis loss will be zero in presence of a steady and time-invariant electric stress. So, on this count also, the losses are more with ac applications. Hence, for the same temperature rise of the insulation, the cable conductor may be allowed to carry more dc current.

⇒ No voltage will be induced in the sheath in de cables and hence sheath losses due to induced current will be zero. However, there will be an insignificant loss taking place due to leakage current. The skin effect will also be absent reducing copper losses. These reductions of losses allow further increase in load current in de cables. Due to these reasons, the current-carrying capacity is increased by about 10-20%.

Answer.4. Any of the above

The various insulating materials used in cables are as follows:

1. Rubber
2. Vulcanized India Rubber (VIR)
3. Impregnated Paper
4. Poly Vinyl Chloride (PVC)
5. XLPC
6. Gutta-percha
7. Silk and Cotton
8. Enamel Insulation
9. Varnished cambric

Answer.4. Maximum at the conductor and minimum at the sheath

Electrostatic stress in an underground cable

The maximum potential gradient occurs at the surface of the conductor and the minimum at the sheath. The potential gradient at any point at a distance x from the center of the conductor is

G = V/rlog_e(R/r) volts/meter

where R is the inner radius of the sheath and r is the radius of the conductor.

Answer.2. Compressed SF6 gas

The transmission which involves voltages in the range of 1000 kV, 1100 kV, 1200 kV, and above is termed as Ultra high voltage transmission line.

Because of the good thermal characteristic and high dielectric strength of the gas SF6, it is used for insulating the cables also. SF 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.

Answer.1. Rubber

Rubber is a natural or synthetic vulcanizable high polymer having high elastic properties. Electrical properties of rubber depending on the degree of compounding and vulcanizing.

There are polymeric materials that exhibit elastic properties similar to those of rubber. The most commonly used polymeric materials are called elastomers. The most commonly used elastomers for high voltage applications are silicone rubber, ethylene-propylene rubber (EPR), and ethylene-propylene dien monomer (EPDM).

Silicone rubber insulation can be safely used in the temperature range of -55°C to 200°C. Silicone rubber exhibits good resistance to ozone, corona, and changing weather conditions. It also has good resistance to alcohol, dilutes acids, alkalies, and almost all types of oils and waxes.

High temperature vulcanized (HTV) silicone rubber is being used in the manufacture of outdoor high voltage insulators. It is used in shipping and aircraft cables, transformers, lightning equipment, etc.

Answer.3. Blue

Three core flexible cords use the color code as follows:- brown for active, light blue neutral and green or green/yellow for earth.

Answer.4. All of the above

The advantages of oil-filled cables over solid cables are as follows:

1. The thickness of the required dielectric medium decreases, therefore the overall size and weight of the cable is lesser.
2. There is no possibility of the formation of voids, oxidation, and ionization within the cable.
3. Impregnation is more perfect.
4. The thermal resistance of the cable reduces and, as a result, the current rating of the cable increases.
5. The maximum allowable stress increases.
6. There is the possibility of an increased working range of temperature for these cables. 7. As soon as the lead at any section deteriorates, the oil starts leaking. Therefore, the possibility of detection of fault is easy.