Generation, Transmission and Distribution of Electrical Power: 17 MCQs with answers

D · Mechanical energy into electrical energy

A generator uses electromagnetic induction to convert mechanical energy (from a turbine) into electrical energy.

C · To reduce the current and thereby reduce $I^2R$ losses

For a given power, higher voltage means lower current. Since line loss is $I^2R$, lower current reduces losses.

D · 220 kV

Transmission voltages are typically 132 kV, 220 kV, 400 kV, and 765 kV. 230 V and 400 V are distribution voltages; 11 kV is primary distribution.

A · It raises the generator voltage to transmission voltage

A step-up transformer increases voltage at the sending end so that power can be transmitted at high voltage.

D · 400 V three-phase / 230 V single-phase

Distribution transformers step down to 400 V three-phase or 230 V single-phase for consumer use.

A · True

Doubling the transmission voltage reduces resistive line loss to one quarter for the same power delivered.

B · $V_L = \sqrt{3}V_{ph}$

In a star connection, line voltage is $\sqrt{3}$ times phase voltage, and line current equals phase current.

A · It becomes one quarter

Doubling voltage halves current for constant power. Since loss is proportional to current squared, loss becomes $(1/2)^2 = 1/4$ of its original value.

B · Transmission is meshed for reliability; distribution is mostly radial

Transmission networks use multiple paths (meshed) for reliability, while distribution networks are usually radial for simplicity and cost.

A · It produces a rotating magnetic field for self-starting motors and saves conductor material

Three-phase systems save conductor material, enable self-starting induction motors, and provide two voltage levels from one supply.

B · They are cheaper than copper and provide high strength with good conductivity

ACSR (Aluminium Conductor Steel Reinforced) combines aluminium's conductivity with steel's strength, making it suitable for long spans and high tension.

D · Transmission uses ACSR on tall towers with large clearances; distribution uses smaller conductors on poles or underground cables

Transmission lines use ACSR conductors on tall towers with large clearances for high voltage, while distribution uses smaller conductors on poles or underground cables.

C · To compensate for voltage variation along feeders

Tap changers adjust the transformer turns ratio to maintain proper voltage levels despite variations along feeders.

C · Line current

In a delta connection, $V_L = V_{ph}$ and $I_L = \sqrt{3}I_{ph}$. So line current is $\sqrt{3}$ times phase current.

D · The 11 kV feeder fault affects many consumers, while the 230 V fault affects few

A higher-voltage feeder fault interrupts supply to a whole feeder (many consumers), whereas a low-voltage service fault affects only a few premises. Protection zones are designed around this.

C · 72.7%

Overall efficiency is the product: $0.95 \times 0.90 \times 0.85 = 0.72675$, or about 72.7%.

B · 62.5 A

To find the current in the transmission line, we use the formula for power: \( P = VI \). Rearranging gives \( I = rac{P}{V} \). At the generator, \( I_{gen} = rac{25 imes 10^6 ext{ W}}{11 imes 10^3 ext{ V}} = 2272.73 ext{ A} \). After stepping up to 400 kV, the current in the transmission line is \( I_{trans} =…