UPPCL JE 2016 Electrical question paper with Solution (Evening Shift)

In air circuit breakers the atmospheric pressure air is used as an arc extinguishing medium. The principle of high resistance interruption is employed for such types of breakers. The length of the arc is increased using arc runners which will increase its resistance in such a way that the voltage drop across the arc becomes more than the supply voltage and the arc will be extinguished. This type of circuit breaker is employed in both A.C and D.C types of circuit breakers upto 12 kV.

Application:

The air blast circuit breakers are preferred for arc furnace duty and traction systems because they are suitable for repeated duty.

A further application of air circuit-breakers is for use with D.C. supplies, this method of interruption still being the most suitable for D.C circuits. D.C. circuit-breakers are widely used in traction applications where ratings of up to some 3 kV may be employed. These type of circuit breakers are finding their best application in systems operating in the range of 132 kV to 400 kV with breaking capacities upto 7000 MVA.

Requirement of DC Ammeter’s Shunt

1. The shunt resistances should be stable and constant with time.
2. The type of material used to join the shunts should have a low thermal dielectric voltage drop.
3. The temperature coefficient of the shunt and the meter should be low and should be as equal as possible.
4. The shunt resistances should not carry currents that will cause excessive temperature rise.
5. The manganin is usually used for the shunts of d.c. instruments while the constantan is useful for the shunts of a.c. instruments.
6. The shunt resistances should be stable and constant with time.
7. The resistances should have a low thermal electromotive force with copper. Since high electromotive force can create undesirable noise during electrical measurements.

A family of four-layer pnpn devices like silicon-controlled rectifier (SCR), silicon controlled switch (SCS), gate turn-off switch (GTO), light-activated SCR (LASCR) with a control mechanism are commonly referred to as thyristors.

BJT comes in the transistor family having three layers, NPN or PNP, and are available as individual components, or fabricated in integrated circuits.

Small rating motors mean which can draw lower current ratings for the desired loads. In the case of delta 3 phase connections, voltages of phase & lines are the same, but currents in line are greater than in phase, & in star connected systems, line voltages being more than phase values, the current is the same for line & phase.

In a star-connected, low rating, three-phase induction motor, the voltage applied to each phase of the stator winding is (1/(√3)) times line voltage. Therefore, if the line voltage is said 415 V, the voltage applied to each phase of the stator winding is 240 V.

An instantaneous over-current relay is one in which no intentional time delay is provided for operation. In such a relay, the relay contacts close immediately after the current in the relay coil exceeds that for which it is set.

Such relay has a unique advantage of reducing the time of operation to a minimum for faults very close to the source where the fault current is the greatest. The instantaneous relay is effective only where the impedance between the relay and source is small compared with the impedance of the section to be protected.

Dielectric heating, also known as electronic heating, RF (radio frequency) heating, and high-frequency heating, is the process in which a high-frequency alternating electric field, radio wave, or microwave electromagnetic radiation heats a dielectric material.

Dielectric heating (also called High-frequency capacitive heating) is employed for heating insulators like wood, plastics, ceramics, etc.

Charged stored in the capacitor is given by

Q = CV

Q = 0.04 × 4 = 0.16 C

Surge impedance is the impedance offered to the propagation of a voltage or current wave during its travel along the line. The approximate value of surge impedance for overhead lines is 400 – 600Ω, for underground cables it is 40 – 60Ω and the transformers have several thousand ohms.

Magnetostriction ( electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ.

Avalanche Breakdown:

With an increasing reverse bias voltage, the electric field across the junction of a p-n diode increases. At a certain reverse bias, the electric field imparts sufficiently high energy to a thermally generated carrier crossing the junction. This carrier, on colliding with a crystal ion on its way, disrupts a covalent bond and produces a new electron-hole pair.

These carriers can also gain sufficient energy from the applied field and collide with other crystal ions to generate further electron-hole pairs. The process is cumulative and produces an avalanche of carriers in a very short time. An Avalanche breakdown is sometimes called an Impact breakdown.

The avalanche breakdown takes place in a junction having a wide depletion layer. With a rise in temperature, the crystal ions vibrate with greater amplitude, increasing the possibility of collisions of the carriers and the crystal ions. The carriers thus have less opportunity to pick up enough energy between collisions to trigger the avalanche process.

Consequently, the avalanche breakdown voltage (which is higher than 6 V) increases with a rise in temperature. The reverse bias voltage is applied is relatively higher of the order of 8 – 10 V depending upon doping. Thus the temperature coefiicient of the avalanche breakdown voltage is positive. The change of the breakdown voltage is about 0.1 percent per centigrade degree change of the diode temperature.