SESSION:3

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1)    What is electrical diversity factor in electrical installations?

Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

2)    Why continuous humming sound occurred in HT transmission line?

This continuous humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

3)    What is different between resistance grounding system and resistance earthing system?

Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.

4)    Why should be the frequency 50 Hz or 60Hz only. Why not others like 45, 95 56 or anything, why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?

We can have the frequency at any frequency you like, but then we must also make your own motors, high voltage transformers or any other equipment we want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50 /60 Hz and the equipment are made to operate at these frequency.

5)    Which type of oil is used as a transformer oil?

Transformer oil or insulating oil is usually a highly refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Well into the 1970s, polychlorinated biphenyls (PCBs) were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

6)    If we give 2334 A, 540 V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 kV?

As we know the Voltage & current relation for transformer,

V1/V2 = I2/I1
We Know, V1 = 540 V; V2=11 kV or 11000 V; I1= 2334 Amps.
By putting these value on Relation,

540/11000= I2/2334

So, I2 = 114.5 Amps

7)    What are the points to be consider for MCB (miniature circuit breaker) selection?

I(L)*1.25=I(Max) maximum current. MCB specification are done on maximum current flow in circuit.

8)    In Three Pin Plug 6 A, 220 V AC rating, why earth pin diameter is higher than other two pin? What its purpose?

Because of larger cross sectional area more current will flow. So if any short circuits occur in the system first high currents bypassed in the Earth terminal. (R=Pl/a area of the conductor increases resistance value decreases, P is resistivity) 

9)    What is the function of interposing Current Transformer? 

The main function of an interposing Current Transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.  Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required. 

10) What are Motor Generator Sets and explain the different ways the motor generator set can be used?

       Motor Generator Sets are a combination of an electrical generator and an engine mounted                   together     to form a single piece of equipment. Motor generator set is also referred to as a                   Genset, or more            commonly, a Generator. The motor generator set can used in the following        different ways: 

a)    Alternating current (AC) to direct current (DC)

b)    DC to AC

c)     DC at one voltage to DC at another voltage

d)     AC at one frequency to AC at another harmonically related frequency

11) What is power quality meter?

Power Quality Meters are common in many industries. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem, or high speed communication lines.

Power Quality Meter gives a graph of Current versus time, Voltage versus time and it shows the percentage of 3^rd, 5^th harmonics in the power supply. Basically it gives the idea of all the parameters of power which determine its quality.

12) What is the different between Digital Phase Converter and Ordinary Phase Converter?

Digital phase converter are a recent development in Phase Converter Technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

13) Explain the operation of variable frequency transformer?

A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly fed electric machine resembling a vertical shaft hydroelectric generator with a three phase wound rotor, connected by slip rings to one external AC power circuit. A direct current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks.

14) What is the main use of rotary phase converter?

Rotary phase converter will be converting single phase power into true balanced 3 phase power, so it is often called as single phase to three phase converter. Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

15) How to determine capacitor tolerance codes?

In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three digit number (such as 130Z).The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico Farads.

16) Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

4-20 mA is a standard range used to indicate measured values for any process. The reason that 4 mA is chosen instead of 0 mA is for fail safe operation .For example, a pressure instrument gives output 4 mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument like broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

17) Two bulbs of 100 W and 40 W respectively connected in series across a 230 V supply which bulb will glow bright and why?

Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb (P=V^2/R). So the resistance of 40 W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

18) What is meant by knee point voltage?

Knee point voltage is calculated for Current Transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated. (CT: Current Transformer).

19) What is reverse power relay?

Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

20) What will happen if DC supply is given on the primary of a transformer?

Mainly Transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high current will flow through primary side of the Transformer. So for this reason coil and insulation will burn out.