SSC JE 2014 Electrical question paper with solution(Morning-Shift)

Advantage of Bundle Conductor

• Bundled conductors per phase reduce the voltage gradient in the vicinity of the line. Thus reduces the possibility of the corona discharge. (Corona effect will be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium. This is avoided by employing bundled conductors).
• Bundled conductor lines will have higher capacitance to neutral in comparison with single lines. Thus they will have higher charging currents which help in improving the power factor.
• Bundled conductor lines will have higher capacitance and lower inductance than ordinary lines they will have higher Surge Impedance Loading  (Z=(L/C)^1/2). Higher Surge Impedance Loading (SIL) will have higher maximum power transfer capability. 
• With an increase in self GMD or GMR inductance per phase will be reduced compared to single conductor line. This results in lesser reactance per phase compared to an ordinary single line. Hence lesser loss due to reactance drop.

Welding arcs give off radiation over a broad range of wavelengths – from 200 nm (nanometers) to 1,400 nm (or 0.2 to 1.4 µm, micrometers). This includes ultraviolet (UV) radiation (200 to 400 nm), visible light (400 to 700 nm), and infrared (IR) radiation (700 to 1,400 nm).

An injury like flash burn occurs when you are exposed to bright ultraviolet (UV) light. It can be caused by all types of UV light, but welding torches are the most common source. That’s why it is sometimes called ‘welder’s flash’ or ‘arc eye

In field resistance control, the lower the field current in a shunt (or separately excited) dc motor, the faster it turns and the higher the field current, the slower it turns.

Since an increase in field current causes a decrease in speed, there is always a minimum achievable speed by field circuit control. This minimum speed occurs when the motor’s field circuit has the maximum permissible current flowing through it.

If a motor is operating at its rated terminal voltage, power, and field current, then it will be running at rated speed, also known as base speed.

Field resistance control can control the speed of the motor for speeds above base speed but not for speeds below the base speed.

To achieve a speed slower than base speed by field circuit control would require excessive field current, possibly- burning up the field windings.

Time constant τ  in series LR circuit is

τ =L/R = 0.01/1 = 0.01

Under steady state condition, L is short-circuited then

Io = V/R =10A

Now the instantaneous current i(t) is

Instrument transformers are used in conjunction with an ammeter and voltmeter to extend the range of meters. In dc circuit shunt and multipliers are used to extend the range of measuring instruments. The shunt is used to extend the range of the ammeter whereas the multiplier is used to extend the range of voltmeters. But in case of AC circuits instruments, it is not convenient to use shunt and multiplier to extend the range of the ammeter and voltmeter.

The current transformer is used in conjunction with the current measuring device (such as an ammeter, wattmeter, energy meter, etc.). The current transformer is used with low-range ammeters to measure currents in a high voltage alternating current circuit. Its primary winding consists of one or more turns of thick wire connected in series with the line whose current is to be measured.

Energy stored in inductor “E” = 1/2(LI²)

Voltage across inductor E(l) = L di(t)/dt

or  i(t) = 1/L di(t)/dt

= 30/5 x 125/3 = 250A

Energy stored in an inductor = (5 x 250²)/2 = 156.25 kJ

Inductance in solenoid is given as

L =μ_oN²A/l

Where μο =vacuum permeability = 4π x 10^-7 wb/m

N = Number of turns

A = area of cross-section

l = length of the solenoid

L =(μ_oN²πr²)/l

Energy stored =  LI²/2

E = (29.43 x 10^-4 x 10²) /2

=0.14715 J ≅ 0.15 J

The plant capacity factor is the measure of the reserve capacity of the plant. A power station must be designed in such a way that it has some reserve capacity for meeting the increased load demand in the future. Therefore the installed capacity of the plant is always greater than the maximum demand on plants.

Reserve capacity = Plant capacity – Max. Demand

If the plant capacity is equal to Max. Demand then the Reserve capacity will be zero.

Shaded pole motor is small in size and less expensive to manufacture. These motors are also called as fractional KW motors because most of these motors are constructed in fractional kilo-watt capacity.

There are five conditions that must be met before when two alternators running in parallel.

1. 1. Equal line voltage
   2. Same frequency
   3. Same phase sequence
   4. Same phase angle
   5. Same waveform

We can use 2 alternators of 6 MVA and 4 MVA each instead of using single 10 MVA alternator because it is economical than using a single alternator of the same rating.