Your Answer : (Not Answered)

Correct Answer : B

The theory is Fortescue's theory.

C.L. Fortescue showed in 1918 that any set of unbalanced three-phase phasors can be resolved into three balanced sets: positive sequence, negative sequence and zero sequence. These symmetrical components are the standard tool for analysing unsymmetrical faults.

Slepian's theory and Cassie's theory are theories of arc interruption in circuit breakers: Slepian's is the race between dielectric strength and restriking voltage, and Cassie's is a model of the arc's conductance. Faraday's work is on electromagnetic induction. None of them deals with symmetrical components.

Your Answer : (Not Answered)

Correct Answer : A

The decreasing order of severity is a-b-c-d: three phase, L-L-G, L-L, L-G.

From the stability point of view, a fault is more severe the more it reduces the power that can be transferred across the system during the fault. A three phase fault brings the transfer close to zero. A double line to ground fault reduces it the most of the unsymmetrical faults, a line to line fault less, and a single line to ground fault, the commonest fault, the least.

The other orders place the single line to ground fault or the line to line fault above the double line to ground fault, which reverses the effect of the fault on power transfer.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : D

Bundled conductors increase the capacitance of an EHV line.

A bundle of two or more sub-conductors per phase behaves like one conductor of much larger effective radius (geometric mean radius). A larger effective radius reduces the line's inductance and increases its capacitance. The lower reactance and higher capacitance raise the surge impedance loading, and the reduced surface field cuts corona loss and radio interference, which is the main reason EHV lines are bundled.

Inductance falls, not rises, and capacitance rises, not falls. Resistance is not increased; for the same total cross-section it stays about the same.

Your Answer : (Not Answered)

Correct Answer : B

As the load factor increases, the cost per kWh decreases.

The cost of generation has a fixed part (interest and depreciation on the plant, which depends on its capacity) and a running part (fuel, which depends on the energy generated). A higher load factor means the same plant produces more energy over the year, so the fixed charges are spread over more units and the cost per unit falls.

The fuel cost per unit stays roughly the same, so there is nothing to make the overall cost rise, and it does not stay unchanged because the fixed cost per unit falls.

Your Answer : (Not Answered)

Correct Answer : B

The overall efficiency is 27.6%.

The overall efficiency of a steam station is the product of its thermal efficiency (heat to mechanical work) and its electrical efficiency (mechanical to electrical energy).

\[ \begin{aligned} \eta_{overall} &= 0.30 \times 0.92 \\ &= 0.276 = 27.6\% \end{aligned} \]

Overall efficiency can never exceed the lower of the two stage efficiencies, which rules out 55.8%, 62.8% and 45%.

Your Answer : (Not Answered)

Correct Answer : D

The source of power is unlimited in a solar power plant. The sun's energy is renewable, and will be available for as long as the sun shines, so it is the inexhaustible source among the options.

Diesel, steam (which burns coal or other fuel) and nuclear plants use fuels that are limited in quantity: oil, coal and uranium. So the fourth option is correct.

Your Answer : (Not Answered)

Correct Answer : D

The reflector of a nuclear reactor is made of beryllium (graphite is also used). A reflector surrounds the core and sends back the neutrons that would escape, so that fewer neutrons are lost.

Boron is used to absorb neutrons in control rods, so it would remove neutrons and not reflect them. Cast iron and steel are structural materials and absorb neutrons. So the fourth option is the answer.

Your Answer : (Not Answered)

Correct Answer : C

The chimney produces a natural draught. The hot gases in the chimney are lighter than the cool outside air, so the difference in the weight of the two columns makes the gases rise through the chimney and draws air through the furnace, without a fan.

Induced and forced draught are made by fans, and a balanced draught uses both. So the third option is correct.

Your Answer : (Not Answered)

Correct Answer : A

The statement that the size of the conductors will increase is not true for high voltage transmission. For a given power, \(I = P/V\), so a higher voltage means a smaller current, and the conductor cross section needed to carry it (and so its weight and cost) decreases. The other three statements are true: the \(I^2R\) losses fall, efficiency rises as a result, and the voltage drop (and so the regulation) is better for the same power delivered.

Your Answer : (Not Answered)

Correct Answer : B

The power transferred through a line is \(P = \dfrac{V_1V_2}{X}\sin\delta\), and for the same level of compensation a series compensator, a capacitor in series with the line, reduces the effective reactance \(X\) directly and so raises the power limit most effectively. A shunt compensator mainly keeps the mid point voltage near its rated value, which also helps but gives less increase for the same size, and a phase angle regulator changes the angle \(\delta\) rather than the reactance. A voltage source inverter is a building block, not a compensator type. The answer is the series compensator, option B.

Your Answer : (Not Answered)

Correct Answer : A

The key gives \(60^\circ\). For a thyristor controlled reactor the fundamental susceptance depends on the firing angle \(\alpha\) through

\[ \begin{aligned} B(\alpha) \\ &= \frac{1}{X}\left(\frac{2(\pi - \alpha) + \sin 2\alpha}{\pi}\right) \end{aligned} \]

The full reactor at 220 V and 1 ohm would absorb \(220^2/1 = 48.4\) kVA, and the stated power of 48.4 kW is that full value, which corresponds to full conduction. The conduction angle in the item is given in a different convention from the one used here, so I could not reproduce the \(60^\circ\) independently. The explanation follows the key, option A.

Your Answer : (Not Answered)

Correct Answer : C

The maximum power that a line can transfer is inversely proportional to its reactance. A series capacitor reduces the net reactance, and with a degree of compensation \(k\)

\[ \begin{aligned} X_{net} \\ &= X(1 - k),\qquad P_{max} \propto \frac{1}{X(1 - k)} \end{aligned} \]

For 75 per cent compensation, \(k = 0.75\):

\[ \frac{P'_{max}}{P_{max}} = \frac{1}{1 - 0.75} = 4 \]

So the maximum power transferred becomes four times the original. The power would be doubled at 50% compensation. The answer is option C.

Your Answer : (Not Answered)

Correct Answer : A

Underground cables are buried to protect them from damage, and where they cross a road they have to be deeper because of the weight and the vibration of the traffic. The key gives the desired minimum depth from the ground level to the top of the cable at road crossings as 1 metre, option (A). The greater depths of 2, 3 and 4 metres are more than needed and would add to the cost of digging without improving the protection.

Your Answer : (Not Answered)

Correct Answer : D

Cables are classed by the voltage they are made for: low voltage cables are up to 1 kV, high tension cables up to 11 kV, super tension cables up to 33 kV, extra high tension cables up to 66 kV and extra super voltage cables beyond 132 kV, as the key gives. So extra super voltage cables are made to operate beyond 132 kV, option (D). The values of 11 kV, 33 kV and 66 kV are the upper limits of the lower classes of cables.

Your Answer : (Not Answered)

Correct Answer : C

Impregnated paper is a cable insulation with high insulation resistance, high dielectric strength and low capacitance, which are its advantages. Its disadvantage is that it is hygroscopic: it absorbs moisture from the air if the lead sheath is damaged, and the moisture lowers its insulation level, so the cable must be sealed. The other three options are advantages and not disadvantages. The answer is hygroscopic nature, option (C).

Your Answer : (Not Answered)

Correct Answer : A

The power transferred is \(P_e = P_{max}\sin\delta\). Before the fault \(P_{max} = 2\) pu and the load is 1 pu.

\[ \begin{aligned} \sin\delta_0 = \frac{1}{2} \Rightarrow \delta_0 \\ &= 30^\circ = \frac{\pi}{6} = 0.5236\ \text{rad} \end{aligned} \]

The fault reduces the limit to 0.5 pu, which is what sets the swing afterwards, but the initial angle only depends on the pre fault values. The other options are larger angles of 36, 42 and 48 degrees. The answer is 0.523 rad, option A.