Electrical Engineering Objectives on Electrical Transmission and Distribution

Current Cut-off Characteristics - Recovering voltage Characteristics of HRC fuse

How A Prime Movers Works In An Electic Power Plant....?

How A Prime Movers Works In An Electic Power Plant....?
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Electrical Power Plant today is more important than any other, that is because without those power plant operate we cannot do anything. From manufacturing plant, because we are all depend on Electricity. So therefore it a high time to know from the very beginning what is a prime movers, how it works or what is
all about.  To get the plant started, prime movers are typically Diesel Engines, Gas or Steam Turbines, or  Hydro and Wind Turbines. This Prime movers converts oil, gas, coal, wood, uranium, water, wind, etc. into mechanical energy. The mechanical energy is supplied to the shaft  of the generator and the generator in turn transform this mechanical energy to electrical energy. High quality electric power generating systems deliver precise voltage and frequency. The voltage regulator as noted earlier, provides the precise voltage regulation. The prime mover GOVERNOR, by accurately  controlling the speed, provides the precise frequency regulation.
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What is a Governor?
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Govers Define as a device which controls the speed or power of a prime mover in order to convert the power to useful work.When the governor senses the speed of a prime mover and controls the prime mover to maintain its speed (or load) at a desired output.Like for example if  you drive a car. You function as a governor when you control the car’s speed under a varying driving conditions. The driver adjust the accelerator that controls the fuel to maintain the desired speed at a certain level and adjust from time to time should the desired speed varies. Acutually the original  governors were entirely mechanical like a pair of centrifugal flyweights in most ballheads. It is known for its ruggedness, usefulness and sensitivity.
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The  flyweights are rotated by a drive from the prime mover that is directly related to the speed. As the speed changes, the flyweights move out at the top due to  centrifugal force. The centrifugal action causes the weights to increase the force on the thruster that control the pilot valve thereby changing the speed of  the prime mover due to applying or removing load.
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What a Governor Does?
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As the prime mover gets started or a speed of about 25%, a governor mechanism “comes to life”. The governor senses the prime mover rotation and begins to  regulate the linkages controlling the fuel/steam supply of the prime mover.
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But if the prime mover tends to increase speed above the set speed of the  governor, the governor mechanism decreases the controlling lever opening of the fuel/steam supply of the prime mover.Eventually, the prime mover speed will  be controlled so that only enough fuel/steam is being admitted, (power in) to hold the prime mover approximately the speed setting of the governor.

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What Happens as Load is Added to the System?
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As the customer load demand increases, the generator must supply the load instantaneously. The generator slows down as more power is being taken out from the  generator than what is being produced. But as soon as the shaft slows down, the governor mechanism senses and causes to increase the fuel/steam rack opening  it and increasing fuel/steam supply of the prime mover.The power produced by the prime mover will increase to match the generator demand. As the speed will  be a little less than before, since if the speed recover completely, the governor mechanism senses and causes to increase the fuel rack opening increasing
power for the prime mover.
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Therefore, the frequency of the AC produced by the generator will get lower as the generator slows down and load increases. What is a Governor Speed Setting Does? The speeder spring is the part that sets the “desired speed”. Applying more force down on the speeder spring causes the governor to increase fuel/steam supply to the prime mover. This initial force is usually set by the operator for the  desired or “reference’ speed.
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It can be set by an adjusting screw, a knob, a lever, a DC electric motor, an air pressure

All About Electrical Conductors Most Frequent Questions

All About Electrical Conductors Most Frequent Questions

What is Conductor ac Resistance....?
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Ans:- A conductor offers a greater resistance to a flow of alternating current than it does to direct current. This increased resistance is generally expressed as the ac/dc resistance ratio. The two major factors for this increase are the skin effect and the proximity effect of closely spaced current carrying conductors. Other magnetic effects can also cause an additional increase in ac/dc resistance ratios.

What is Electric Fields and Voltage...?
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Ans:- Current flow is charge in motion. We might consider the simple case of a conductor carrying current out to a load and then a return conductor as two separated parallel cylinders of charge. If we neglect conductor diameter line of charge there are electric field lines represented by circles of diameters such that the center of the circles are on the 0 line and each circle passes through the center of the cylinders.

What is Conductor...?
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Ans:- Conductors may be solid or stranded. Metals used are commonly copper or aluminum. An attempt to use sodium was short-lived. The strand can be concentric, compressed,compacted, segmental, or annular to achieve desired properties of  flexibility, diameter, and current density.

What is Circular Mil Sizes...?
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Ans:- Sizes larger than #4/0 AWG are specified in terms of the total cross-sectional area of the conductor and are expressed in circular mils. This method uses an arbitrary area of a conductor that is achieved by squaring the diameter of a solid conductor. This drops the π/4 multiplier required for the actual area of a round conductor.
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A circular mil is a unit of area equal to the area of a circle having a diameter of one mil, one mil equals 0.001
inch. Such a circle has an area of 0.7854 or π/4 square mils. Thus, a wire 10 mils in diameter has a cross-sectional area of 100 circular mils. Likewise, one square inch equals 4/π times 1,000,000=1,273,000 circular mils. For convenience, this is usually expressed in thousands of circular mils and abbreviated kcmil.

What is Non-Shielded Power Cable...?
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Ans:- A cable non-shielded cable may consist of one or several conductors and one or several
insulating layers.
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The cable may contain a jacket. The cable may also include a conductor
shield.
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A cable is not considered fully shielded until both conductor and insulation shields
are present.
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Non-shielded cables are common in the 0 to 5 kV voltage range although
non-shielded power cables through 8 kV have been available.

What is AIR INSULATED CONDUCTORS...?
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Ans:- A metallic conductor suspended from insulating supports, surrounded by air, and carrying electric signals or power may be considered as the simplest case of an insulated conductor.

What is INSULATING TO SAVE SPACE...?
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Ans:- Space is a common constraint that precludes the use of air as an insulator. Imagine the space requirements to wire a house or apartment using bare conductors on supports with air as the insulation. A voltage divider has been created that is made up of the impedance from the conductor to the outside covering surface and another impedance from the covering surface to ground. The distribution of voltage from conductor to the surface of the covering and from the covering surface to ground will be in proportion to these impedances.

What is RISING VOLTAGE...?
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Ans:- Return to the metallic conductor that is covered with an insulating material and suspended in air. When the ground plane is brought close or touches the covering,At low voltages, the effect is negligible. As the voltage increases, the point is reached where the potential gradients are sufficient to cause current to flow across the surface of the covering.

What is Electrical Insulation or Dielectric...?
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Ans:- The insulation dielectric provides sufficient separation between the conductor and the nearest electrical ground to adjacent phase to preclude dielectric failure. For low voltage cables, (2,000 volts and below), the required thickness of insulation to physically protect the conductor is more than adequate

Electrical machines interview questions and answers

[1] Define the term synchronous speed [Dec-2003]
For synchronous machines there exists a fixed relationship between number of poles P, frequency (f) and the speed of the machine. The speed of the synchronous machine for the given number of poles and the rated frequency is called the synchronous speed mentioned as NS.

[2] Write an expression for synchronous speed (Dec-2004)
An expression for synchronous speed is NS = 120f/P.
Where
f = frequency
P = No of poles of the machine.

[3] What does speed voltage mean? (Dec-2007)
When the magnetic flux is constant as well as stationary and the coil rotates to cut the flux then EMF gets induced due to relative speed between flux and coil. This EMF is called speed EMF, rotational EMF or dynamically induced EMF.

[4] Mention the factors on which hysteresis loss depends (Dec-2008)
The hysteresis loss is directly proportional to the area under the hysteresis curve ie area of the hysteresis loop.
It is directly proportional to frequency ie number of cycles of magnetization per second.
It is directly proportional to volume of the material.

[5] Distinguish between statically induced and dynamically induced EMF (Dec - 2010, 2011, 2009)
How is EMF induced dynamically ( May-2010)
An induced EMF which is due to physical movement of coil, conductor with respect to flux or movement of magnet with respect to stationary coil, conductor is called dynamically induced EMF or motional induced EMF.
The change in flux lines with respect to coil can be achieved without physically moving the coil or the magnet. Such induced emf in a coil which is without physical movement of coil or a magnet is called statically induced EMF.

[6] Define torque (May-2010)
A turning or a twisting force about an axis is called as torque.

[8] What are the three types of basic rotating electric machines? (May-2011)
DC machines
Induction Machines
Synchronous Machines

[9] What are the causes of core loss? what are the components of core loss?
When a core is subjected to an alternating flux then it undergoes the cycles of magnetisation and demagnetisation. This produces hysteresis effect which causes hysteresis loss in the core.
Similarly core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, EMF gets induced in the core. Such currents in the core which are due to induced emf in the core are called as eddy current loss. Thus eddy current and hysteresis are the two components of the core loss.

What are Transients in a Power System?

What are Transients in a Power System?

Transient can be stated as the phenomenon’ when the change is significant but the duration of the change is not much’. But they are very important for power system because depending upon the intensity of the transient, fault occurrence in a network can be severe or less severe.

Transient phenomenon can be understood more easily by this instance. Any change in system network cannot take place instantaneously, some time is needed for this change. For example voltage build up across a circuit is not instantaneous, it needs some time to reach the peak value, and this period is known as transient period.
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Suppose a network is normally charged and carries load and suddenly a fault occurs. Then the main switch is closed or opened, this redistribution bound upon transient period, depending on which the severity of the fault is determined. Three facts makes this more understandable:

Current cannot change instantaneously.
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Voltage across a load cannot change instantaneously.
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Law of conservation of energy holds good for the compounds which stores energy (Inductors, Capacitors) and the transient phenomenon is widely seen on them.

What is a composite conductor?

What is a composite conductor?

For transmission of high voltages, stranded conductors are used. These are known as composite conductors. These are composed of two or more strands of cable. Mainly for high voltage transmission, stranded copper conductors are used.
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Solid conductors are used before for transmission purpose, but stranded conductors have many more advantages. That’s why they are rapidly used. They are widely used because of their:

• Flexibility.

• Mechanical stability.

• Reduction in skin effect (the tendency of the current to flow through the surface of the conductor).

What is the purpose of step up transformer in electrical power system?

What is the purpose of step up transformer in electrical power system?

The main use of transformer in electrical power system is to produce the needed voltage level. Some time voltage is needed to be increased or decreased than the available voltage level. Depending on the operation, transformers can be divided into two types,

(i) Step up transformer,
(ii) Step down transformer.

As the name suggests, the step up transformers are used to step up the voltage level. These are also termed as power transformer when used in power plants. Similarly step down transformers are used to step down or bring down the voltage.

Now, coming to the step up transformer, they are used in electrical power system at generation level. When power is generated at power plants the voltage level is pretty low.
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So when they are transmitted from generation station to distribution substation, power is reduced due to the different types of losses and the voltage of the receiving end decreases radically and thus the economic purpose is also not served.

That’s why step up transformers are introduced at the power plants to step up the voltage level before transmission. So, the receiving end voltage is not decreased too much.
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Again as this high voltage cannot be used directly, so voltage is again stepped down by step down transformer before distribution.
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Core balanced current transformer

Core balanced current transformer

Core balance current transformer or CBCT is a special variation of current transformer, where the main current coil is constructed in ring style. This type of current transformers are specially used for the measurement of three phase current. They are installed in substations in order to protect the line from earth faults.
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So, an earth fault relay is connected with the CBCT and after sensing the fault current, CT gives the fault signal to the relay. Energised phase cables are inserted through the hollow section of ring coil of CBCT, and automatically the primary of current transformer built, and the air section in between the cable and ring coil acts like the low reluctance flux path.
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In this way, the ring coil becomes the secondary of the Core balance current transformer. In three phase power system, the current in phase cables are balanced. So, during the normal operation conditions, no current is there in the secondary ring coil of CT.
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But, when an earth fault is arised, that is any of the phase cable subjected to the direct contact with earth, then huge amount of unbalanced fault current starts flowing through the conductors.
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When this residual fault current travels through the phase cable, then automatically there will be a considerable current in the secondary coil of CBCT.
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This fault current signal is received by the relay coil, and automatically trips the contacts. In this way, the line is kept protected from the effect of local earth fault.
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                 ( Sai Saikumar Jn)

Magnetic Circuit - electrical MCQ and answers

Magnetic Circuit - electrical MCQ and answers

1. An air gap is usually inserted in magnetic circuits to
(a)     increase m.m.f.
(b)     increase the flux
(c) prevent saturation
(d) none of the above
Ans: c

2. The relative permeability of a ferromagnetic material is
(a)     less than one    
(b)     more than one
(c) more than 10
(d) more than 100 or 1000
Ans: d

3. The unit of magnetic flux is
(a)     henry
(b)     weber
(c) ampereturn/weber
(d) ampere/metre
Ans: b

4. Permeability in a magnetic circuit corresponds to______ in an electric circuit.
(a) resistance
(b) resistivity
(c) conductivity    
(d) conductance
Ans: c

5. Point out the wrong statement.
Magnetic leakage is undesirable in electric machines because it
(a) lowers their power efficiency
(b) increases their cost of manufacture
(c) leads to their increased weight
(d) produces fringing
Ans: a

6. Relative permeability of vacuum is
(a) 1
(b) 1 H/m
(c) 1/4JI
(d) 4n x 10-' H/m
Ans: a

7. Permanent magnets are normally made of
(a) alnico alloys    
(b) aluminium
(c) cast iron
(d) wrought iron
Ans: a

8. Energy stored by a coil is doubled when its current is increased by percent.
(a) 25
(b) 50
(c)41.4
(d) 100
Ans: c

9. Those magnetic materials are best suited for making armature and transformer cores which have____permeability and_______hystersis loss.
(a) high, high
(b) low, high
(c) high, low
(d) low, low
Ans: c

10. The rate of rise of current through an inductive coil is maximum
(a)     at 63.2% of its maximum steady value
(b)     at the start of the current flow
(c) after one time constant
(d) near the final maximum value of current
Ans: b

11. When both the inductance and resistance of a coil are doubled the value of
(a) time constant remains unchanged
(b) initial  rate  of rise  of current is doubled
(c) final steady current is doubled
(d) time constant is halved
Ans: a

12. The initial rate of rise of current through a coil of inductance 10 H when suddenly connected to a D.C. supply of 200 V is_______Vs
(a) 50
(b) 20
(c) 0.05
(d) 500
Ans: b

13. A material for good magnetic memory should have
(a) low hysteresis loss
(b) high permeability
(c) low retentivity
(d) high retentivity
Ans: d

14. Conductivity is analogous to
(a) retentivity
(b) resistivity
(c) permeability    
(d) inductance
Ans: c

15. In a magnetic material hysteresis loss takes place primarily due to
(a) rapid reversals of its magnetisation
(b) flux density lagging behind magnetising force
(c) molecular friction
(d) it high retentivity
Ans: d

16. Those materials are well suited for making permanent magnets which have______retentivity and _______coercivity.
(a) low, high
(b) high, high
(c) high, low
(d) low, low
Ans: b

17. If the area of hysteresis loop of a material is large, the hysteresis loss in this material will be
(a) zero
(b) small
(c) large
(d) none of the above
Ans: c

18. Hard steel is suitable for making permanent magnets because
(a) it has good residual magnetism
(b) its hysteresis loop has large area
(c) its mechanical strength is high
(d) its mechanical strength is low
Ans: a

19. Silicon steel is used in electrical machines because it has
(a)     low co-ercivity
(b)     low retentivity
(c) low hysteresis loss
(d) high co-ercivity
Ans: c

20. Conductance is analogous to
(a) permeance
(b) reluctance
(c) flux
(d) inductance
Ans: a

21. The property of a material which opposes the creation of magnetic flux in it is known as
(a) reluctivity
(b) magnetomotive force
(c) permeance
(d)     reluctance
Ans: d

22. The unit of retentivity is
(a)     weber
(b)     weber/sq. m
(c) ampere turn/metre
(d) ampere turn
Ans: b

23. Reciprocal of reluctance is
(a) reluctivity
(b) permeance
(c) permeability    
(d) susceptibility
Ans: b
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Admin ;- Bellapuri saikumar
                   ( Sai Saikumar Jn)

Electrical interview questions 17

Eee interview questions and answers

Electrical interview question 20

Electrical interview questions and answer's

Electrical shock (Sai saikumar jn)

                                                                    Electrical shock

1) Why We use of Stones/Gravel in electrical Switch Yard....? ( sai saikumar jn )

1) Why We use of Stones/Gravel in electrical Switch Yard..

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 *.Reducing Step and Touch potentials during Short Circuit Faults 

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*.Eliminates the growth of weeds and small plants in the yard 

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*.Improves yard working condition 

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*.Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat. 

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Q:Why is the starting current high in a DC motor? ( sai saikumar jn )

Q:- Why is the starting current high in a DC motor? 

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 Ans:In DC motors, Voltage equation is V=Eb+IaRa (V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current, Ra = Aramture resistance). 

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 At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

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Q:If one lamp connects between two phases it will glow or not? ( sai saikumar jn )

Q:If one lamp connects between two phases it will glow or not? 

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 A:If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp. 

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Q:When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? ( sai saikumar jn )

Q:When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

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 A:No.We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature).

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So,the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire.

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So, for a better protection of the system.It should have both over voltage & over current relay.

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Q:Which power plant has high load factor? ( sai saikumar jn )

Q:Which power plant has high load factor? 

.:All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load,we can reduce the cost of generation. Hydel power plants have a higher efficiency than thermal & nuclear power plants.

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Q:What is the power factor of an alternator at no load? ( sai saikumar jn )

Q:What is the power factor of an alternator at no load? 

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 A:At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor. 

Q: What will happen if DC supply is given on the primary of a transformer? ( sai saikumar jn )

Q: What will happen if DC supply is given on the primary of a transformer? 

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 Ans: 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. 

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 So high electrical current will flow through primary side of the transformer.So for this reason coil and insulation will burn out.

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