UPPCL JE Electrical question paper with Explanation 2018

In a metal like copper or tungsten, there are plenty of free electrons. At room temperature, these electrons wander randomly in the atomic structure, but they cannot leave the metallic surface. At room temperature, ordinary metals do not lose their electrons. This means that a force must exist, which prevents electrons from leaving the metallic surface permanently. Since the electrons of the outermost shell, a metal has sufficient kinetic energy, so these electrons try to be emitted out from the metal surface, but due to attractive forces of nuclei, these electrons cannot escape from the metal surface.

To understand what this force is, and how it is created, let us assume that due to its random motion, an electron leaves the surface. Immediately after it leaves the surface, the metal gains a positive charge (losing a negatively charged electron is equivalent to gaining a positive charge). This positive charge exerts a force of attraction on the emitted electron. This force pulls the electron back to the metal. For an electron to escape from the metal surface, it must have sufficient kinetic energy to overcome this force. This force is described as the surface barrier.

The surface barrier is analogous to the gravitational pull of the earth. If a body is thrown upwards, it comes back to the earth because of the gravitational force. For a rocket or spaceship to come out of the earth’s attracting field, it has to be launched with a velocity greater than a particular value. This value of minimum velocity is called escape velocity. Similarly, an electron can come out of the metallic surface permanently, only if its velocity (or kinetic energy) is more than a particular value. Modern physics tells us that even at the absolute zero of temperature, the velocity (or kinetic energy) of all the electrons does not reduce to zero; there are many electrons that possess appreciable energy. The highest energy that an electron in a metal has, at the absolute zero of temperature is called the Fermi level of energy. It is designated as E_n. For emission to take place, we have to supply additional energy from outside. This additional energy needed for emission is called the work function of the metal. However, if the temperature of the metal is high enough of an order of 150 to 2500 K, some electrons gain sufficient thermal energy to escape out from the metal surface. Thus electrons are emitted from the metal surface which is called thermionic emission. The emission of an electron from a hot metal surface is similar to the discharge of steam from heated water.

The operational amplifier (popularly known as an op-amp) is an active device used to design circuits that perform useful operations, such as generating sine waves or square waves; amplifying, combining, integrating, differentiating and removing noise; and transforming alternating current into direct current and vice-versa. It can also change the shape of a waveform, produce a change in the output when an input signal reaches a certain level, provide constant voltage or current, and perform various other important circuit operations. Op-amp circuits are very important as we develop a valuable perception about how electronic circuits work in general.

An op-amp is a very high-gain differential amplifier with high input impedance and low output impedance.

An ideal op-amp would show the following characteristics:

1. Infinite open loop voltage (A_v = ∞)
2. Infinite input resistance (R_i = ∞)
3. Zero output resistance (R_o = ∞)
4. Infinite bandwidth (BW = ∞)
5. Zero output voltage when input voltage is zero
6. Perfect balance, in zero output voltage when the same voltage is applied at both the inputs
7. Infinite common-mote rejection ratio (CMRR = ∞)
8. Infinite slew rate, in output voltage changes occur simultaneously with input voltage change
9. Stable characteristics against temperature variation.
10. Noise and distortion is zero.

The common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e., those that appear simultaneously and in-phase on both inputs.

it is defined as the ratio of differential voltage gain to common mode voltage gain

CMMR = Ad/Ac

Where 
Ad is Differential voltage gain
and
Ac is common mode voltage gain

ii) what do i mean by common mode voltage(gain)?

Let V₁ and V₂ be input voltages and Vo be output voltage 
 Being an differential amplifier

V_o=V₂ − V₁ …..(also known as differential output)

Common mode basically means that both the inputs are common to OpAmp
therefore, in our case V₂=V₁ so Vo becomes ZERO and thus 

Common mode voltage gain is ZERO (for ideal OpAmp)

As discussed above

CMMR= Ad/Ac

Ac=0 (again, for ideal OpAMP)

therefore CMMR is infinite for ideal OpAmp

A generating station in which nuclear energy is convened into electrical energy is known as the nuclear power plant. In a nuclear power plant, heavy elements such as Uranium (U²³⁵) or Thorium (Th²³²) are subjected to nuclear fission in the nuclear reactor.

Nuclear fission means breaking up of nuclei of heavy atoms into small masses with a huge amount of energy. The release of energy is due to mass defect i.e., the mass of the final product comes to be Icss than the initial product. This mass defect is converted into heat energy according to Einstein’s relation E = mc², where m = mass of the atom and c = velocity of light. Nuclear fission is accompanied by the chain reaction i.e. emission of neutrons continues the process randily. But in a nuclear reactor, the controlled reaction is allowed.

General Arrangement of Nuclear Power Plant

The Fig.shows the schematic arrangement of a nuclear power plant.

The entire arrangement can be divided into the following stages

1. Nuclear reactor

2. Heat exchanger (Steam generator)

3. Steam turbine

4. Alternator

5. Cooling water circuit

Nuclear Reactor

This represents that part of a nuclear power plant where U²³⁵ fuel rods is subjected to a controlled fission chain reaction, during which tremendous energy is generated.

Moderators: The main function of the moderators is to reduce the energy of the neutron evolved during fission. By slowing down the high energy neutron, We possibility of escape of neutrons is reduced while the possibility of absorption of neutrons by fuel to cause further fission is increased. This also reduces the amount of fuel required for the chain reaction.

Heat Exchanger

It is a device which is used to exchange the heat from the primary circuit to the secondary circuit. The coolant carries the heat in the reactor to the exchanger where it is exchanged to the water, to convert water into steam. Thus the heat exchanger is nothing but a steam generator. Once the heat is exchanged, the coolant is fed back to the reactor, using the coolant recirculating pump.

Steam Turbine

The steam generated from the water in the secondary circuit is taken to the steam turbine through the main valve, where it is expanded. Due to this, the turbine starts rotating and thus the heat energy is converted to a mechanical energy.

Alternator

The shaft of an alternator is coupled to the turbine Shaft. Thus when the hue rotates, the alternator starts rotating. The alternator converts mechanical energy into an electrical energy. The energy output of an alternator is given to the busbars through the transformer, circuit breakers and isolator.

Cooling Water Clrcult

The expanded steam from the turbine Is Me exhausted steam which is taken to the condenser. In the condenser, the steam is condensed into water. For the condensation of steam, a flow of natural cold water is circulated through the condenser. This water takes heat from the exhaust steam. This hot water is passed through the cooling tower, where it is again converted to cold water. Then it is recirculated through the condenser by the pump. The condensed steam is then recirculated through the secondary artist of the exchanger, using the feed water pump.

Current Ratings of SCR

The current carrying ability of the SCR is determined by the temperature at its junction. Since the thyristor is made up of a semiconductor material, it’s thermal capacity is, therefore, quite small. Hence, even for short overcurrents, the junction temperature may exceed the rated value and the device may be damaged. In this section, current ratings of SCR are discussed for both repetitive and non- repetitive type of current waveforms.

1.i²t rating or amp² sec rating

The i²t rating is the measure of thermal energy that the device can absorb for a short period of time. Whenever the fault occurs, the fast acting fuse clears such fault. Due to the fault, thermal energy is generated in the device also. The fuse should clear the fault and device should be protected. Hence i²t rating is used to determine about how long the device can absorb the thermal energy. The fuse must clear the fault before the device is damaged due to exceeding i²t rating.

2. Average current rating (I_T)

The average current rating is the maximum repetitive average current that can flow through the SCR. The power loss in the SCR depends upon average current flowing through it. If the SCR is operating at the sufficiently high frequency, then switching loss wilt also be significant. Hence switching losses may be added to loss due to average current.

3.RMS current rating (I_TR)

The RMS current rating is the maximum repetitive RMS current that can flow through the SCR. The RMS current rating is same as an average current rating for DC current. This rating is required to prevent excessive heating in metallic joints, leads, and interfaces of SCRs.

4.Surge current rating (I_TSM)

The surge current rating is the peak amplitude of the surge current that the SCR can withstand only for the limited number of times in its life cycle. The surge current is normally specified as the number of cycles and peak amplitude. The SCR may be damaged when surge current rating and its number of cycles are exceeded.

5. di/dt rating

The di/dt rating specifies the maximum allowable rate of change of current through the device. Due to rapid variations in anode current, the carriers do not spread across the junctions at the turn-on time. Hence they are concentrated in a small area of the device, creating local heating. This is called hot-spot created due to high current density in the restricted area of the junctions. Because of this, the junction temperature increases and the device may be damaged. The di/dt rating specifies maximum allowable variations in anode current so that the device will not be damaged. Normally it is specified in Amperes/microseconds and typical values are from 50 A/μs to 800 A/μs sec.

6. Voltage Ratings

The SCR blocks the forward and reverse voltages. The voltage ratings mainly specify the maximum allowable voltages those the device can withstand without damaging the junctions.

7. dv/dt Rating

The dv/dt rating specifies the maximum allowable rate of change of forward voltage that the device can withstand in forward direction. If the forward voltage variations exceed dv/dt rating, then the device turns on. Such turn on is false triggering and disturb the operation of the controller.

Nowadays, almost all alternators are connected it parallel with other alternators. The satisfactory parallel operation of alternators depends upon the synchronizing power. If the synchronizing power is higher, the higher is the capability of the system to synchronism.

The variation of synchronous power with the change of load angle is called the synchronizing power. It exists only during the transient state, i.e. whenever there is a sudden disturbance in load (or steady-state operating conditions). Once the steady state is reached, the synchronizing power reduces to zero.

If an alternator transfers power to an infinite bus at a steady-state power angle δ_oand a sudden transient disturbance due to increase in load will occur the rotor of the alternator will accelerate and at a load angle δ_o + dδ, the alternator supplies new power P + dP. Therefore, the operating point shifts to a new line. The steady-state power input remains unchanged. Therefore, the additional load decreases the speed of the machine and hence the alternator comes back to the steady-state position. Similarly, due to sudden transient, if the rotor retards, the load angle will decrease. The operating point will shift to a new line and the load on the machine becomes P – dP. Once again the input to the machine remains unaltered, therefore, the rotor is accelerated due to the reduction in load. Hence, the machine comes back to synchronism. Therefore, we can conclude that the effectiveness of this correction action depends upon the change in power transfer for a given change in load angle

The synchronizing power flows from or to the bus in order to maintain the relative velocity between an interacting stator and rotor field, zero, once the equality is reached, the synchronizing power vanishes.

The synchronizing power coefficient, which is defined by the rate at which synchronous power varies with load angle (S) gives a measure of effectiveness. The synchronizing power coefficient is also called stiffness of coupling, rigidity factor or stability factor. It is denoted by P_syn.

The synchronizing power of the synchronous machine is given as

Where

V = supply Voltage

E_b = Back E.M.F

X_s = Synchronous Reactance

Hence from the above expression, it is clear that synchronizing power is inversely proportional to the synchronous reactance.

Load factor is the ratio of average load to the maximum demand during a given period.

Load factor = Average Load/Maximum Demand

Plant capacity factor is defined as the ratio of the average load to the rated capacity of the power plant ie. the aggregate rating of tile generators. It is preferable to use continuous rating while calculating the aggregate.

Plant capacity factor = Average load/Plant capacity

Reverse capacity is the difference between Plant capacity and Max.demand

Reverse capacity = Plant capacity – Max.demand

Given data

Maximum demand = 50 MW

Load factor = 60% = 0.6

Plant capacity factor = 0.5

Average load = Max.demand × Load factor

Average load = 50 × 0.6 = 30 MW

Plant capacity factor = Average load/Plant capacity

Plant capacity = Average load/Plant capacity factor

Plant capacity = 30/0.5 = 60 MW

Reverse capacity = Plant capacity – Max.demand

Reverse capacity = 60 – 50 = 10 MW

For common emitter, the current gain β is defined as the ratio of collector current to base current at a constant V_CE .

β = I_C/I_B

For common-base dc current gain (α) is defined as the ratio of the collector current, I_c, and the emitter current I_E and it is represented by α. The dc current gain α can be expressed as

α = I_C/I_E

Relation between α & β

β = α ⁄ (1 − α)

Given

Collector current I_C = 100.2mA

Emitter current I_E = 100mA

α = I_C/I_E = 100.2/100 = 1.002

β = α ⁄ (1 − α) = 1.002/(1 −1.002)

β = −501

Here negative just gives information about its direction and nothing else. Because by convention positive current is always defined as flowing into the device. So if you have a PNP common base amplifier and you source current into the emitter input, it will flow out of the collector output. Since the current is flowing out, it is a negative current, hence the gain is negative.

To enable the synchronous machine to start independently as a motor, a damper winding is made on rotor pole face slots. Bars of copper, aluminum, bronze or similar alloys are inserted in slots made or pole shoes as shown in Fig. These bars are short-circuited by end-rings or each side of the poles. Thus these short-circuited bars form a squirrel-cage winding. On application of three-phase supply to the stator, a synchronous motor with damper winding will start at a three-phase induction motor and rotate at a speed near to synchronous speed. Now with the application of dc excitation to the field windings, the rotor will be pulled into synchronous speed since the rotor pole are now rotating at only slip-speed with respect to the staler rotating magnetic field.

Use of Damper winding to prevent Hunting

Hunting:

Sudden changes of load on synchronous motors may sometimes set up oscillations that are superimposed upon the normal rotation, resulting in periodic variations of a very low frequency in speed. This effect is known as hunting or phase-swinging.

During Hunting, the rotor of the synchronous motor starts to oscillate in its mean position, therefore, a relative motion exists between damper winding and hence the rotating magnetic field is created. Due to this relative motion, e.m.f. gets induced in the damper winding. According to Lenz’s law, the direction of induced e.m.f. is always so as to oppose the cause producing it. The cause is the hunting. So such induced e.m.f. oppose the hunting. The induced e.m.f. tries to damp the oscillations as quickly as possible. Thus hunting is minimized due to damper winding.

During starting period when the speed of the synchronous machine is less than the synchronous speed the damper winding acts as the Induction motor providing Induction motor torque. Now during hunting the speed of the synchronous machine become more than the synchronous speed now the damper winding will act as an induction generator and it will generate the induction generator torque to counteract the loss in the synchronism.

An ideal resistor dissipates (converts into heat) electrical power. They are not capable of delivering power. Capacitors and inductors both are capable of absorbing and delivering (positive) power. When power is absorbed by an ideal capacitor, all of it is stored in the form of an electric field. Likewise, all of the power absorbed by an ideal inductor is stored in the form of a magnetic field. These devices can deliver this stored energy, but cannot produce energy. They never dissipate energy, stores energy during the one-half cycle and releases energy during another half cycle for an AC signal. (when you switch on them they will ‘store’ energy in them. But when you switch off the supply theoretically you can get back energy that was stored.)

Real capacitors and inductors, however, are not ideal and will dissipate some power due to imperfections within the device (leakage within a capacitor, for example). This is why in simulations, capacitors and inductors will sometimes have very complex models to attempt to simulate real-world behavior (such as a leakage within a capacitor, which can be modeled simply with a high-resistance resistor in parallel with the capacitor).

A silver-oxide battery non-rechargeable is a primary cell with a very high energy-to-weight ratio.they are known both as button cells, for example, for hearing aids, watches, or cameras, and for military applications, usually as reserve batteries.