SSC JE 2011 Electrical question paper with Solution

During Arc extinction process SF6 is decomposed into atoms, electrons and ions. These atomic components do not recombine completely to form the original SF6 gas on cooling. They form low molecular gaseous sulfur fluorides and compounds with the contact metals principally SF4 and SF2, together with a small amount of  S2, F2, S, and F.

Switch arc essentially can be treated as a capacitor with the contacts as the plate. When the contacts are far apart there is a very small capacitance, but as the switch closes the capacitance increases rapidly causing a significant change in the amount of charge stored in the “plates” this current surge is the source of the spark.

When capacitors are connected together in parallel the total or equivalent capacitance, CT in the circuit is equal to the sum of all the individual capacitors added together. This parallel capacitor will create an alternate current path for transient currents rather than jumping the closing gap of the switch contacts, the current will take the path of least resistance into the capacitor. Therefore the low value of surge occurs hence no arc will generate.

Characteristic Impedance of the line

Z = √L/C

and Z = V/I

1. Melting point of Eureka is 1221 to 1300 °C
2. Melting point of Kanthal is 1,425 °C
3. Melting point of Nichrome is 1400 °C
4. Molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications as pigments and catalysts. The melting point of platinum-molybdenum carbon compound is around 3000°C

The generator used for welding purposes should have high current and low voltage which can be obtained by the differential compound generator.

Differentially compounded DC generator has drooping characteristics that mean if load current increases the net flux will decreases(i.e shunt field and series fields in opposition will increases)due to the demagnetization effect hence the Induced  EMF and terminal voltage decreases.

Let the transistor be in active region then by applying Kirchoff’s law to the base-emitter loop,

10 – I_B (310 × 10³ ) – 0.6 = 0

I_B = 30.3225 mA

I_c  = βI_B

= 125 × 30.3225 × 10^–6 = 3.79 mA

Now VCE = 20 – IcRc

= 20 – 5 × 10³ (3.79 × 10^–3 )

= 1.05V

The line losses are inversely proportional to the square of voltage and power factor.

P_L ∝ 1/V²

Now voltage is doubled

V₂ = 2V^₁

P_L1/P_L2 = (V₂/V^₁)²

= (2V_¹/V_¹)²

P_L2 = P_L2/4

Capacity Factor

The capacity factor indicates the extent of the use of the generating station. It is different from the load factor because of the reason that the rated capacity of each plant is always greater than the expected maximum load due to some reserve capacity. Thus

Utilization Factor

It is defined as the ratio of maximum demand to the rated capacity of the plant

 Leading kVAr taken by the capacitor,

Q = P(tanΦ₁ – tanΦ₂)

CosΦ₁ = 0.707

Φ₁ = 45°

CosΦ₂ = 0.866

Φ₂ = 30°

Q = 17.32 (tan 45° – tan 30°)

= 7.32 KVAR

Transposition of overhead line conductors refers to exchanging the positions of power conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor over an equal distance in order to reduce crosstalk and otherwise improve transmission.

Usually, the telecommunication line and power lines travel close to each other, then the current flowing through the power line produces magnetic flux linkage between the telecommunication line so that voltage is induced in the telecommunication line which causes the disturbance in communication. This phenomenon is called radio interference which is suppressed by the transpose of the power line.

The transposition arrangement of the conductor can simply show in the following figure. The conductor in Position 1, Position 2, and Position 3 changes in a specific arrangement to reduce the effect of capacitance and the electrostatic unbalanced voltages.