Insulators and Underground Cables Questions and Answers

Answer: C

The gas is Sulphur hexafluoride (SF₆).

SF₆ is electronegative: it captures free electrons, so its dielectric strength is about two to three times that of air at the same pressure. It is non-flammable and chemically stable, and it is widely used in circuit breakers, gas-insulated switchgear and gas-insulated lines.

Nitrogen and carbon dioxide have been used as insulating gases in the past but are far weaker. Freon has good dielectric strength but is used as a refrigerant, not as the standard insulating medium in electrical apparatus.

Answer: C

Electromechanical breakdown occurs due to Mechanical stresses produced by the electric field.

A solid dielectric between two electrodes is squeezed by the electrostatic attraction between them. If the material is soft enough, the compressive force reduces its thickness, the field rises, the force rises further, and the material collapses mechanically before its intrinsic electrical strength is reached.

Magnetic hum and vibrations are mechanical effects unrelated to the field across the insulation. Voltage fluctuations produce electrical stress, not the mechanical compression this breakdown mechanism depends on.

Answer: D

In a thyrite arrester the resistance is high at low current and low at high current.

Thyrite is a silicon carbide material with a strongly non-linear voltage-current characteristic. At normal system voltage it offers a very high resistance and passes almost no current, so the line is not loaded. When a lightning surge raises the voltage, its resistance collapses and it passes the large surge current to earth, then recovers its high resistance once the voltage returns to normal.

The other options describe a resistance that rises with voltage, the opposite of what an arrester needs: a resistor that grew with the surge would block the discharge instead of passing it.

Answer: D

The stress is maximum at the conductor surface and minimum at the sheath.

In a single-core cable the electric field at radius \(x\) in the dielectric is \(g = \frac{V}{x \ln(R/r)}\), where \(r\) is the conductor radius and \(R\) the inner sheath radius. The stress therefore varies inversely with \(x\).

It is greatest at the smallest radius, the conductor surface, and least at the largest radius, the sheath. This is why cable insulation is designed for the stress at the conductor, and why grading is used to even it out.

The options putting the maximum at the sheath reverse the relationship, the stress cannot be uniform because the field spreads out with radius, and it is never zero at the conductor surface.

Answer: A

The insulators used are of the strain type.

At a dead end, a sharp change of direction or a long river crossing, the line exerts a large horizontal pull on the tower. Strain insulators, strings of discs (or, at lower voltage, shackle units) mounted roughly horizontally in line with the conductor, are designed to take that mechanical tension.

Suspension insulators hang vertically and carry the conductor's weight on straight runs. Pin insulators are used on straight runs of lines up to about 33 kV, and shackle insulators mainly on low-voltage distribution lines.

Answer: D

The surge impedance of an underground cable is around 50 Ω.

Surge impedance is \(Z_{0} = \sqrt{L/C}\). In a cable the conductors sit close together inside a sheath with a dielectric of high permittivity, so the inductance per kilometre is small and the capacitance large, and \(Z_{0}\) comes out low, typically 40 to 60 Ω.

About 400 Ω is the figure for an overhead line, where wide spacing in air gives high inductance and low capacitance. 100 Ω and 25 Ω are outside the usual range for power cables.

Answer: D

At river and road crossings strain type insulators are used.

Where a line crosses a river or road it is usually dead-ended on stronger towers, with long spans and the conductors under heavy tension. Strain insulators, which are discs or long-rod units mounted horizontally in line with the conductor, are built to take that mechanical pull. They are also used at dead ends and sharp bends.

Suspension insulators hang vertically on straight runs and carry the conductor's weight rather than its full tension. Pin type insulators are limited to straight runs at lower voltages, and egg (shackle or stay) insulators are used in guy wires and low-voltage distribution lines.

Answer: C

The earth wire protects the line against lightning surges.

The overhead earth (shield or ground) wire is strung along the tops of the towers, above the phase conductors, and earthed at every tower. A lightning stroke that would otherwise hit a phase conductor strikes the earth wire instead and is conducted to ground through the towers, so the conductors lie within its zone of protection.

Switching surges arise inside the system when breakers operate and are controlled by the breakers' design and by surge arresters. Corona depends on the conductor's size and surface field and is reduced by larger or bundled conductors, not by an earth wire. 'Ensure fault voltages' does not describe a protective function.

Answer: C

Pin type insulators become uneconomical beyond 33 kV.

A pin insulator is a single piece mounted on a pin on the cross-arm. As the voltage rises it must be made larger and heavier, with more sheds and greater thickness, so that above about 33 kV it becomes bulky, costly and hard to make reliably. Above that level suspension insulators, built up from strings of discs, are used instead, because the voltage rating can be raised simply by adding discs.

11 kV is well within the pin insulator's range, and 50 kV and 60 kV are already past the point where it is economical.