Electrical Engineering Exem and Interview Preparation

Q: What is electric traction?

A: Electric traction means using the electric power for traction system (i.e. for railways, trams, trolleys etc.). Electric traction means use of the electricity for all the above machines. Now a day, magnetic traction is also used for bullet trains. And basically dc motors are used for electric traction systems.

Q: How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?

A: It's possible by means of Electronic chokes, otherwise it's not possible to ionize the particles in tube light with normal voltage.

Q: What is "Pu" in electrical engineering?

A: Pu stands for per unit and this will be used in single line diagram of power distribution and it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).

Q: Operation carried out in Thermal power stations?

A: The water is obtained in the boiler and the coal/gas is burnt so that steam is obtained this steam is allowed to hit the turbine, the turbine which is coupled with the generator generates the electricity.

Q: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

A: Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

Q: What is the difference between electronic regulator and ordinary electrical rheostat regulator for fans?

A:The difference between the electronic and ordinary electrical regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved. In electronic regulator, triac is employed for speed control by varying the firing angle speed and it is controlled but in rheostatic, control resistance is decreased by steps to achieve speed control.

Q: How tube light circuit is connected and how it works?

A: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Q: what is MARX CIRCUIT?

A: It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

Q: What is encoder, how it function?

A: An encoder is a device used to change a signal (such as a bit stream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with analog circuitry.

Q: What are the advantages of speed control using thyristor?

A: Advantages: 1. Fast Switching Characteristics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.

Q: Why Human body feel Electric shock? and in an Electric train during running, We didn’t feel any Shock ? why?

A: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Q: what is the principle of motor?

A: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

Q: Why, when birds sit on transmission lines or current wires doesn't get shock?
A: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded. So that means that human who touch the line closes the circuit between phase and neutral.

Q: what is meant by armature reaction?

A: The effect of armature flux to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Q: what happen if we give 220 volts dc supply to a bulb or tube light?

A: Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

A: DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.

Q: what is ACSR cable and where we use it?

A: ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission & distribution.

Q: What is vacuum circuit breaker? Define with cause and where could be use this Device?

A: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure the arc formation is interrupted. VCB's can be used up to 11kv.

Q: What will happen when power factor is leading in distribution of power?

A: If there is high power factor, i.e. if the power factor is close to one:

1. Losses in form of heat will be reduced,

2. Cable becomes less bulky and easy to carry, and very cheap to afford, &

3. It also reduces over heating of transformers.

Q: What is 2 phase motor?

A: A two phase motor is a motor with the starting winding and the running winding have a phase split. e.g. ac servo motor. Where the auxiliary winding and the control winding have a phase split of 90 degree.

Q: What is the significance of vector grouping in Power Transformers?

A: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. EG. DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.

Q: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc.) which are find in the houses?

A: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Q: Give two basic speed control scheme of DC shunt motor?

A: 1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux, speed can be controlled 2.armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (I_aR_a)can be varied, and since speed is directly proportional to E_b-I_aR_a the speed can be controlled.

Q: What is the difference between synchronous generator & asynchronous generator?

A: In simple, synchronous generator supply's both active and reactive power but asynchronous generator (induction generator) supply's only active power and observe reactive power for magnetizing. This type of generators are used in windmills.

Q: What is the Polarization index value? (pi value) and simple definition of polarization index ?

A: It is ratio between insulation resistance (IR) i.e. meggar value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5.

Q: Why syn. generators are used for the production of electricity?

A: Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence syn. generators are used for the production of electricity.

Q: 1 ton is equal to how many watts?

A: 1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower, 12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW.

Q: Enlist types of dc generator?

A: DC Generators are classified into two types 1) separately excited dc generator 2) self-excited dc generator, which is further classified into;1)series 2)shunt and 3)compound(which is further classified into cumulative and differential).

Q: What is Automatic Voltage regulator (AVR)?

A: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators, it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Q: What is an exciter and how does it work?

A: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator. Rotary exciter is an additional small generator mounted on the shaft of main generator. If it is dc generator, it will supply dc to the rotary poles through slip ring and brushes (conventional alternator). if it is an ac exciter, output of ac exciter is rectified by rotating diodes and supply dc to main fixed poles. ac exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.

Q: Why use the VCB at High Transmission System? Why can't use ACB?

A: Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the die electric strengths equal to 8 times of air. That why always vacuum used as in HT breaker and air used as in LT.

Q: What is the difference between surge arrestor and lightning arrestor?

A: LA is installed outside and the effect of lightning is grounded, whereas surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q: What happens if I connect a capacitor to a generator load?

A: Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Q: Why the capacitors works on ac only?

A: Generally capacitor gives infinite resistance to dc components (i.e. block the dc components). It allows the ac components to pass through.

Q: Explain the working principal of the circuit breaker?

A: Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

Q: How many types of cooling system in transformers?

A: 1. ONAN (oil natural, air natural)

2. ONAF (oil natural, air forced)

3. OFAF (oil forced, air forced)

4. ODWF (oil direct, water forced)

5. OFAN (oil forced, air forced)

Q: What is the function of anti-pumping in circuit breaker?

A: when breaker is close at one time by close push button, the anti-pumping contactor prevent re close the breaker by close push button after if it already close.

Q: What is stepper motor? What is its uses?

A: Stepper motor is the electrical machine which act upon input pulse applied to it. it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used.

Q: How to calculate capacitor bank value to maintain unity power factor with some suitable example?

A: KVAR= KW(TAN(COS^(-1)#e)- TAN(COS^(-1)#d) )

#e= EXISTING P.F.

#d= DESIRED P.F.

Q: Detail about c.t. and p.t.?(Company: reliance)

A: The term C.T means current transformer, and the term P.T means potential transformer. In circuit where measurements of high voltage and high current is involved they are used there. Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device. so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on leakage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed. E.g. if CT is of ratio 5000/5A and it has to measure secondary current of 8000A.then ANS=8000*5/5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current. Same is the operation of PT but measuring voltage.

Q: There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?

A: The motor has max load current compare to that of transformer because the motor consumes real power. And the transformer is only producing the working flux and it’s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

Q: what is power factor? Whether it should be high or low? Why?

A: Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more. It is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping. (If pf is poor ex: 0.17 to meet actual power load has to draw more current (V constant), result in more losses if pf is good ex: 0.95 to meet actual power load has to draw less current (V constant),result in less losses).

Q: What is the difference between Isolator and Circuit Breaker?

A: Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operated. its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

Q: what is boucholz relay and the significance of it in to the transformer?

A: Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

Q: What is SF6 Circuit Breaker?

A: SF6 is Sulpher hexa Flouride gas. if this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

Q: what is ferrantie effect?

A: Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

Q: what is meant by insulation voltage in cables? Explain it?

A: It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.

Q: Why we do 2 types of earthing on transformer (ie:)body earthing & neutral earthing , what is function.

A: The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment earthing: body (non-conducting part)of the equipment should be earthed to safegaurd the human beings. system Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted.so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment (transformer or alternator) and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

Q: What is the difference between MCB & MCCB, Where it can be used?

A: MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Q: Where should the lighting arrestor be placed in distribution lines?

A: Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.

Q: Define IDMT relay?

A: It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense, the tripping time will decrease as the magnitude of fault current increase.

Q: What are the transformer losses?

A: TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I₂R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.

Query: What is meant by regenerative braking?

Resolution: When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply. So that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as "Regenerative Breaking".

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

Resolution: In DC motors, Voltage equation is V=E_b-I_aR_a (V = Terminal voltage, E_b = Back emf in Motor,I_a = Armature current,R_a = Aramture resistance).At starting, E_b is zero. Therefore, V=I_aR_a, I_a = V/R_a , where Ra is very less like 0.01ohm.i.e, I_a will become enormously increased.

Query: What are the advantages of star-delta starter with induction motor?

Resolution: (1). The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting. (2). Hence the starting current is reduced, the voltage drops during the starting of motor in systems are reduced.

Query: Why Delta Star Transformers are used for Lighting Loads?

Resolution: For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Query: Why series motor cannot be started on no-load?

Resolution: Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

Query: Why ELCB can't work if N input of ELCB do not connect to ground?

Resolution: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

Q: Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?

A: Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. so, interchanging of terminals in ac system does not show any difference. That's why the ac solenoid attract the plunger even though its terminals are interchanged

Q: What is derating? why it is necessary, it is same for all means for drives, motors, and cables.

A: The current currying capacity of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called derating.

Q: Why temperature rise is conducted in bus bars and isolators?

A: Bus bars and isolators are rated for continuous power flow that means they carry heavy currents which rises their temperature. so it is necessary to test this devices for temperature rise.

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

Q: How do you select a cable size (Cu & Al) for a particular load?
A: At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalog considering derating electrical current. After that measure the length of cable required from supply point of load to load poin. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop>3% then choose next higher size of cable.

Q: What are HRC fuses and where it is used?

A: HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

Q: Which power plant has high load factor?
A: 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.

Q: Mention the methods for starting an induction motor?
A: The different methods of starting an induction motor
DOL: direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter

Q: What is the difference between earth resistance and earth electrode resistance?
A: Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away. The resistance of the electrode has the following components:
(A) the resistance of the metal and that of the connection to it.

(B) the contact resistance of the surrounding earth to the electrode.

Q: What is use of lockout relay in ht voltage?
A: A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Q: What is the power factor of an alternator at no load?
A: At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

Query: How to determine capacitor tolerance codes?

Resolution: 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.

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

Resolution: 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) 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.

Query: Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

Resolution: 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 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

Query: What is meant by knee point voltage?

Resolution: Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated. (CT-current transformer).

Query: What is reverse power relay?

Resolution: Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to fed 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.

Query: What will happen if DC supply is given on the primary of a transformer?
Resolution: 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 electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out.

What is transposition of a transmission line?

The balancing effect on account of irregular spacing of conductors can be avoided by changing the position of the conductors at regular distances. This is called transposition of conductors. In case of 3- phase transmission lines, the inductance and capacitance of each phase will be different if the three conductors are spaced differently (each phase to the other two phases, and each phase to ground). The apparent resistance of the conductors is also affected on account of transfer of power between the phases which occur due to mutual coupling. So all the three parameters of the transmission lines are affected by irregular spacing of the conductors.

Alternator/Generator vector diagram

Synchronous motor vector diagram

Transformer Diagram

Power plant

Unit generated per day = Max. demand (KW) × Load factor × 24 hours
Units generated / annum = Max. demand × Load factor × Hours in a year
Demand factor = (Max. demand) / (connected load)
Average demand = (units generated per annum) / (hours in a year)
Load factor = average demand / Max. demand
Plant capacity factor = average demand / Plant capacity = (Units generated per annum) / (Plant capacity × Hours in a year)
Reserve capacity of plant = Plant capacity – Max. demand
(Straight line method) Annual depreciation charges = (P-S) / n, P = initial cost of equipment's, n = useful life, S = Salvage or scrap value
(Sinking fund method) Annual deposit, q = (P-S)[r / {(1+r)ⁿ - 1}], r = compound interest, Sinking fund at the end of m^th years = q {(1+r)^m -1}/r
Demand charge = tariff on maximum demand × maximum demand
Energy charge = tariff on energy consumption × annual consumption
Annual bill = Demand charge + Energy charge
Flat rate of energy consumption = Flat rate / unit = (annual bill) / (annual consumption)
1 calorie = 4.1858 Joules
Overall efficiency = Thermal efficiency × Electrical efficiency
η_overall= (Electrical output in heat units) / (Heat of combustion)
Thermal efficiency = η_boiler× η_turbine
For hydroelectric plant, Electrical energy available = W × H × η_overall, W = mg
For hydroelectric plant, the volume of water which can be utilized per annum = Catchment area × Annual rainfall × Yield factor
For hydroelectric plant, work done / sec. = W × H = gross plant capacity
Firm capacity = Plant efficiency × gross plant capacity
For hydroelectric plant, output per annum = Firm capacity × Hours in a year
For hydroelectric plant, overall efficiency, η_overall = η_penstock × η_turbine × η_generator
Diversity factor = (Sum of individual max. demand) / (Max. demand on power plant)
Daily average load = (No. of units generated in a day) / (24 hours)
1 BTU(British Thermal Unit) = 1053 Jouls
1 KWh = 3418 BTU
Efficiency of steam power station is about 29%
From the time the power station is installed, its equipment steadily deteriorates due to wear and tear so that there is a gradual reduction in the value of the plant. This reduction in the value of plant every year is known as depreciation. Due to depreciation, the plant has to be replaced by the new one after its useful life. Thus while determining the cost of production of electrical energy, annual depreciation charge must be included.

Active power = VA cos θ
Reactive power = VA sin θ
Q = √(S² – P²), Q = reactive power, S = apparent power, P = active power
Power factor = (active power) / (apparent power)
For fixed P and V_L, P.F ∞ 1 / I_L,so, I_L ↓ P.F ↑]

Alternators

Balanced earth-fault protection is generally provided for small size alternators.
Overheating caused by overloads, short-circuit, failure of cooling systems.
The most dangerous fault in an alternator is the stator winding fault.
Faults of alternator – 1. Failure of prime mover 2. Failure of field 3. Over current 4. Over speed 5. Over voltage 6. Unbalanced loading 7. Stator winding faults
Protection of alternator – 1. Differential protection 2. Balanced earth fault protection 3. Stator inter-turn protection
Inverted running of alternator => When input to the prime-mover fails, the prime-mover stops the conversion of mechanical energy into electrical energy. If the alternator works in parallel with other alternators, then it will start working as a synchronous motor running at synchronous speed. This motoring condition is known as inverted running.

Synchronous motor

Rated synchronous motors at 150KW (200 H.P) to 15 MW (20000H.P) - speeds-> 150 rpm to 1800 rpm.
For synchronous motors, supply frequency in cycles per second = the rotor revolves at the same cycles per second.
Synchronous motor is a doubly fed motor where torque is non-zero at only one speed called synchronous speed.
If the mechanical angle α between the stator and rotor poles increases, then stator current is increased.
Use of Synchronous motors: Synchronous motors are rarely used below 50 H.P. because of their much higher initial cost compared to 3-phase induction motors. However, their unique characteristics of constant speed operation, power-factor control and high operating efficiency make them highly suitable for heavy industry, particularly for applications requiring low speed and high horsepower.
1. A very common application is for driving air or gas compressors. It is especially desirable that compressors be driven at constant speed since their output and efficiency vary considerably with the speed at which they are operated.
2. Another important application is for driving large DC generators. The synchronous motor does the additional job of improving the p.f of the system. Note: In addition to the above mentioned cases, synchronous motors are also prominently found in the following application:
  1. Large, low head pumps
  2. Flour-mill line shafts
  3. Rubber mills and mixers
  4. Crushers
  5. Chippers
  6. Centrifugal pumps
  7. Rolling mills
  8. Pulp grinders
  9. Frequency changers etc.
Hunting of a synchronous motor- Although the speed of a synchronous motor is constant, it often tends to oscillate or hunt around its synchronous speed. This hunting is caused due to (1) change in load (2) change in excitation (3) change in some other condition in the system. Hunting results in rapid change of mechanical angle between the rotor and stator poles and corresponding variations of stator current. If hunting becomes too severe, either the circuit of the motor will be opened by the circuit breaker or the motor will pull-out of synchronism and come to a halt. Hunting is reduced by damper winding provided on the rotor of the motor.

Transmission and Distribution

For very long high voltage transmission lines – over current protection is distance protection.
For greater length and exposure to atmosphere overhead line have more chance of faults occurring.
½ Li² = ½ Cv²
To eliminate voltage surge required, R = ½ √(L/C)
% reactance at Base KVA = {(Base KVA) / (Rated KVA)} × (% reactance of rated KVA)
Short circuit current, I_sc = I × {100 / (% reactance)}, I = Full load current
% resistance of line = (Base KVA × Resistance of line) / {10 × (KV)²}
Zero Phase Sequence Current in each Phase = 1/3 [current in neutral wire]
Positive Sequence Impedance = Negative Sequence Impedance
Zero Phase Sequence Component of Current (R-phase), I_R0 = 1/3 [I_R + I_Y + I_B]
Positive Phase Sequence Component of Current (R-phase), I_R1 = 1/3 [I_R + α I_Y + α² I_B]
Negative Phase Sequence Component of Current (R-phase), I_R2 = 1/3 [I_R + α² I_Y + α I_B]
I_R0 = I_Y0= I_B0
I_R= I_R0+ I_R1 + I_R2
I_Y= I_R0+ α²I_R1 + αI_R2
3-wire DC system makes 2 voltages, (1) Between the neutral and any of the two outers (2) Between the outers.
In interconnected systems feeder ring is energized by more then 1 substation so it is the most reliable.
Insulation resistance, R = (ρ/2πL) log_e(r₂/r₁), r₁= inner radius, r₂= outer radius, L = length of the cable. Thus R∞ (1/L)
g_max/ g_min = D/d, g= stress, d = core diameter, D = internal sheath diameter
g_max = 2v / {d log_e(D/d)}
Cable Capacitance, C = [ε_rl / {41.4 log₁₀(D/d)}] × 10^-9F
Insulation capacitances are ineffective for DC, so string efficiency is 100%
Voltage across string = Phase Voltage
String Efficiency = {(Voltage across string) / (No. of insulators × V₃)} × 100, V₃ = Voltage across the string nearest to the conductor.
For shunt capacitance decreases voltage across various discs tend to be uniform, thus V₃ decreases and string efficiency increases.
Sag, S = wl²/ 8T, w=weight per meter length of the conductor, l = length of span, T = tension
S ∞ 1/T, S is provided to reduce T
X₁ = (l / 2) – (Th/wl), w = weight per meter, T = Tension in the conductor X₂ = (l / 2) + (Th/wl)
Working tension, T = Working stress × Area of conductor
Density of Ice = 0.9 × 10³ Kg/m³
Volume of ice per meter length of conductor = π/4 [(D + 2t)² – D²] × 1 = πt (D + t) t = thickness of ice on conductor
For transmission, area of cross-section of conductor ∞ 1/V²
For transmission, volume of conductor material (also cost of conductor material, c), v ∞ 1 / (V²cos²θ), for P.F unity, C or v ∞ 1 / V²
Energy lost per annum(annual) = 2I²Rt / 1000 KWh, R = Resistance of the wire, t = time in hours in a year
Annual cost of energy lost = cost per KWh × annual energy loss
Annual charge = annual interest and depreciation on capital cost of cable.
According to Kelvin’s law, for most economical cross-section of the conductor variable annual charge = Annual cost of energy lost
The disadvantage of a radial feeder by using definite time relay is that if there are a number of feeders in series, tripping time for faults near the supply end become high. This is undesirable because the safety of the equipment and other components in line of the system will be endangered.
Symmetrical components do not have separate existence. They are only mathematical components of unbalanced currents or voltages which do exist in the system.
The choice of transmission voltage mainly depends on the following factors:
1. Length of transmission line
2. Amount of power to be transmitted
3. Percentage voltage regulation required
4. Cost of switchgear, terminal equipment and conductor material.
An electric supply system consists of three principle components viz the power station, the transmission lines and the distribution system.

Switchgear

The operating time of relay should be less then 1 second.
Isolators open a circuit under no load condition.
Current chopping occurs at air-blast circuit breaker for low currents.
Air-blast circuit breaker- High pressure air-blast is used for extinguishing the arc. The contacts are opened in a flow of air-blast established by the opening of blast valve. The air blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly increased the dielectric strength of the medium between contacts and prevents from reestablishing the arc. Consequently the arc is extinguished and flow of current is interrupted.
The rated normal current = Current rating of the CB
Rated symmetrical breaking current of a CB = (VA rating)/(√3 × Voltage rating)
Breaking Capacity = VA rating of the CB
Rated making current = 2.55 × Rated symmetrical breaking current
Low oil CB has small amount of oil only sufficient for arc extinction.
Breaking capacity of SF₆does not depend on weather and moisture, pollution and atmospheric pressure etc.
Pick-up current = Rated secondary current of C.T × Current Setting
Plug Setting Multiplier (P.S.M) = (Fault Current in relay coil) / (Pick-up current)
For fuse wire – I²∞ r³
The operating time of a fuse = 0.002 sec. and for CB = 0.2 sec.
Peak Re-striking Voltage occurs at time, t = 1 / (2f_n), t = time for 1^st peak.
Average Rate of Rise of Re-striking Voltage, (RRRV) = (Peak re-striking voltage) / (time to reach the peak)
The fusing current is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it.
The fusing current depends upon the various factors such as:
1. Material of fuse element
2. Length-the smaller the length, the greater the current because a short fuse can easily conduct away the heat.
3. Diameter
4. Size and location of terminals
5. Previous history
6. Type of enclosure used
Circuit Breaker => Circuit Breakers are designed to interrupt either normal or short-circuit currents. They behave like big switches that may be opened or closed either manually or automatically. A circuit breaker will automatically open a circuit whenever the line current exceeds a preset value.

Digital logic

Boolean algebra- (1) X+X’ = 1 (2) X+1 = 1 (3) (X’)’ = X (4) (X+Y)’ = X’ Y’ (5) X+XY = X (6) X.1 = X (7) X.X’ = 0 (8) X.X = X (9) X+YZ = (X+Y)(X+Z) (10) (XY)’ = X’ + Y’ (11) X(X+Y) = X

RS Flip-flop-

S	R	Q	Q'
0	0	1	0 (after S=1, R=0)
			1 (after S=0, R=1)
0	1	0	1
1	0	1	0
1	1	0	0

Electronics

Transistor current gain, β = I_C / I_B
α = I_C / I_E
**Common collector connection is primarily used for impedance matching purpose. Because of its high input impedance and low output impedance.
In case of Darlington Pair, β = β₁ β₂

Induction motor

For induction motor, (rotor output) / (rotor input) = 1 – S

Circuit

1 Coulomb = Charge on 625 × 10¹⁶ electrons.
Active element => Supplies electrical energy to the circuit.
Passive element => Receives electrical energy and then converts it into heat or store in an electrical field or magnetic field.
Nodes => Two or more circuit elements are joined.
Junctions => Three or more circuit elements are joined.
Branch => Lies between two junction points.
Loop => Closed path of a network.
Mesh => Elementary form of a loop which cannot be further divided into other loops.
Reciprocity Theorem => In any linear bilateral network, if an emf E acting in a branch X causes a current I in branch Y, then the same emf E located in branch Y will causes a current I in branch X.
1 H.P = 746 watts.
Work done per second = 2πNT/60 watts, N=> R.P.M, T=> Torque
Charge on capacitor during charging, q = Q (1 – e^-t/RC), Q = Max. Charge
Voltage on capacitor during charging, v = V (1 - e^-t/RC)
During discharging, q = Q e^-t/RC
During discharging, I = I₀ e^-t/RC
Paramagnetic materials => Relative permeability slightly greater then 1 and much less then 1.1 => Aluminum and Platinum.
Diamagnetic Materials => Relative permeability slightly less then 1. => Carbon, Copper and Silver etc.
Ferromagnetic materials => Relative permeability much greater then 1.
Lenz’s law => The induced current will flow in such a direction so as to oppose the cause that produced it.

Transformer

All day efficiency, η_all-day = (KWH output in 24 hours) / (KWH input in 24 hours)
Protection provided to a distribution transformer-

Air break switch (gang-operating switch) with a fuse is provided on the 11 KV side of the transformer for isolation and protection respectively.
LT switch with a fuse is provided on the LT side of the transformer for isolation and protection against feeder faults.

History of electricity in Bangladesh

1^st electric lamp in Dhaka 7/12/1901
Electricity generation capacity of Bangladesh 5560mw
Electricity requirement in Bangladesh 6066mw
In 2008-2009 per head daily production of electricity is 172 KWH
In 2008-2009 Max. electricity production in Bangladesh is 4130 mw
Kaptai hydro electric power plant established in 1962
Ruppur nuclear power plant- 1961
Rural Electrification Board established in 1977
Khulna barge-mount power plant capacity 110mw
Bijoyer alo from Labuan iland of Malaysia 3/8/1999 –130mw
Bd system loss-6.62%
DESA system loss 14.26%
Meghna ghat elect. 450mw largest in bd
1^st gas elect horipur, Shilet
1^st coal elect bor pukuria, dinaj pur
New 4 elect- 1. Narayan gonj shiddirgonj 210mw 2. Shylhet shahji bajar 60mw 3. Gazipur tongi 80mw 4. Dinajpur borpukuria 250 mw

#Economizer
Economizers are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. The term economizer is used for other purposes as well. In simple terms, an economizer is a heat exchanger.
#Solar cell working principle and raw materials
Traditional explanation
The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons emitted, but does not increase the energy that each electron possesses. Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons.
Raw materials
The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame.
#Globalization

Globalization describes an ongoing process by which regional economies, societies, and cultures have become integrated through a globe-spanning network of communication and trade. The term is sometimes used to refer specifically to economic globalization: the integration of national economies into the international economy through trade, foreign direct investment, capital flows, migration, and the spread of technology.
#Global warming

Carbon dioxide and other air pollution that is collecting in the atmosphere like a thickening blanket, trapping the sun's heat and causing the planet to warm up.
Global warming doesn't create hurricanes, but it does make them stronger and more dangerous. Because the ocean is getting warmer, tropical storms can pick up more energy and become more powerful. So global warming could turn, say, a category 3 storm into a much more dangerous category 4 storm.
Melting glaciers, early snowmelt and severe droughts will cause more dramatic water shortages in the American West.
Rising sea levels will lead to coastal flooding on the Eastern seaboard, in Florida, and in other areas, such as the Gulf of Mexico.
Warmer sea surface temperatures will fuel more intense hurricanes in the southeastern Atlantic and Gulf coasts.
Forests, farms and cities will face troublesome new pests and more mosquito-borne diseases.
Disruption of habitats such as coral reefs and alpine meadows could drive many plant and animal species to extinction.
The United States produce 25 percent of the carbon dioxide pollution from fossil-fuel burning -- by far the largest share of any country. In fact, the United States emits more carbon dioxide than China, India and Japan, combined.

#Name of the liquid mixture in lead-acid battery

Electrolytes
#Use of Capacitors
1. Energy storage
2. Pulsed power and weapons
3. Power conditioning
4. Power factor correction
5. Supression and coupling
6. Decoupling
7. Noise filters and snubbers--When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.
8. Motor starters
9. Signal processing-The energy stored in a capacitor can be used to represent information, either in binary form, as in DRAMs, or in analogue form, as in analog sampled filters and CCDs. Capacitors can be used in analog circuits as components of integrators or more complex filters and in negative feedback loop stabilization. Signal processing circuits also use capacitors to integrate a current signal.
10. Tuned circuits
11. Sensing
#BCD Code
1. To BCD-encode decimal number using the common encoding, each decimal digit is stored in a four-bit nibble.
Decimal: 0 1 2 3 4 5 6 7 8 9
BCD: 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
2. Thus, the BCD encoding for the number 127 would be:
Decimal: 1 2 3
BCD: 0001 0010 0111
# Conditions for Parallel operation of transformer-

Primary windings of the transformers should be suitable for the supply system voltage and frequency.
The transformers should be properly connected with regard to polarity.
The voltage rating of both primaries and secondaries should be identical (same turn ratio or transformation ratio).
The percentage impedance should be equal in magnitude and same X/R ratio in order to avoid circulating currents and operation at different power factors.
With transformers having different KVA ratings the equivalent impedances should be inversely proportional to the individual KVA rating if circulation currents to be avoided.

#Conditions for Parallel operation of Alternator-

The terminal voltage (effective) of the incoming alternator must be the same as the bus-bar voltage.
The speed of the incoming machine must be such that its frequency (=120f/p) equals the bus-bar frequency.
The phase of the alternator voltage must be identical with the phase of the bus-bar voltage.

#Alternator Protection
The three main type protection systems are:

over current protection
reverse power trip
under voltage trip

The over current protection works by removing power supplies to non essential services on a preferential basis. This is done with the help of preferential trips which decides the load that is to be removed in case of overloading of the distribution system.
Reverse power trip and under voltage trip are required only if there is more than one alternator on board which requires working of two alternators on load.
#Over-Current Protection
Every alternator has an over current protection. With the help of this trip, the alternator and distribution system can be protected from various faults but the main thing to be considered in this method is to maintain power to the distribution system till the time the alternator trips on any other protection devices.
For this reason, the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by an inverse definite minimum time (IDMT) relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics
But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system. Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow.
If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc., from the switchboard. These preferential trips are operated by relays which are set to about 110% of the normal full load of alternator.
#Inverse Definite Minimum Time (IDMT) Relay
The construction of this type of relay is similar to that of a household watt meter. The actual inverse time delay characteristics are obtained with the help of an induction type relay.
#Reverse Power Protection
There is not much difference between an alternator and electric motors from the engineer's perspective. They are both based on similar principles. So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are running in parallel.
Hence this type of protection system is used only if there is more than one alternator on board a ship. The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs. This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault. Though it is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.
#Programmable logic controller
A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.
The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

#explain power factor in simple words.....
The beneficial part of the power supplied to the circuit that the circuit actually performs work i.e. ratio of real power to apparent power is called power factor
#Which power plant has high load factor?

Nuclear power plant. This plant consumes less energy for generating high power.
#How many types of dc batteries standards are present?

1.2v (Ni-cad) 1.5 v (dry cell), 6v (lead-acid), 9v, 12v (lead-acid)
#How many methods for starting an induction motor?

1. DIRECT ON LINE STARTER (DOL)
2. STAR DELTA STARTER
3. VARIABLE FREQUENCY DRIVES
4. RESISTANCE STARTERS
5. AUTO TRANSFORMER STARTER
6. SERIES REACTOR STARTER
#An electric bulb is connected on a holder. One end of the wire is connected to the phase and the remaining one connected to earth instead of neutral. the bulb will glow or not? if yes then what is the need of neutral?

Yes the bulb will glow, if the earth is pakka. The neutral is earthed at each distribution transformers, form this point only neutral is taken.
#If a capacitor connected to power supply will it consume power & will it going to get recorded in energy meter
This consumption of energy is getting recorded in electronic energy meters but I don’t know it is correct or not
#Can u start a tube light without a starter?

YES! Tube light starter is just like a push button. Not of use once the system is started. When u have switched on the supply in the TL touch and remove the contacts of the 2 terminals that were to be connected to the starter a several times so that u generate appropriate di/dt, until the required voltage is induced in the choke coil that u'll see as light starting glowing
#What is meant by HRC fuses? In the distribution system different rating hrc fuses are used for transformer (which is 3k, 6k, 10k, 15k etc).what is k?

HRC stand for "high rupturing capacity" fuse & k stands for thousand, 3k means 3000 ohm, 5k means 5000 ohm like wise.
#What is the difference between earth resistance and earth electrode resistance?
Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
The resistance of the electrode has the following components:
(A) the resistance of the metal and that of the connection to it.
(B) the contact resistance of the surrounding earth to the electrode.
(C) the resistance in the surrounding earth to current flow or earth resistivity which is often the most significant factor.

#Can anyone explain how the vector diagram of a transformer which doesn't have name plate be found whether it is Dy11/Dy5/Yd11 and so on?

The designation Yy indicates that the upper and under voltage windings have been star-connected. Yd denotes upper voltage winding as star and under voltage winding as delta.
Vector group designation
Vector group = circuit + index
Example:
Yy0 =Y star connection of the upper voltage winding OS,
y star connection of the under voltage winding US,
0×30 degrees = 0 degrees phase displacement
Yah! can.
If Dy11
Primary winding having only 3 terminals with high resistance and secondary having 4 terminals with low resistance.
If Yd11
Primary having 4 terminals with high resistance and
secondary having 3 terminals with low resistance.
If Dy5
This is not a verctor group.
Eppdi???
Ya ! first of all you can decide Yd or Dy by seeing it primary & secondary connections and 1,2,3....11 we can decide it by doing vector group tests int that we can check phase shift of wave form & we can conclude Whether Dy or Yd can be ascertained from the bushings.To find particular vector group ,CRO can be used finding relationship between the current vectors.e.g. In Dy 11 I2R lags I1R by 30 degrees Number 11 denotes position of minute- hand in a clock. It can also found out by taking various voltage measurements and analyzing.
#Why we do occ & scc test in transformer?how we can do it ?
occ test is perfomed to find no load loss of transformer.
scc test is perfomed to find full load loss of transformer.
oc for iron loss
sc for copper loss
#What is Electricity?
FLOW OF ELECTRONS INA CONDUCTOR AGAINST A RESISTANCE
#What is meant by Floating Neutral? How to prevent floating neutral is one whose voltage is not constant?

floating earthing means the neutral is not earth by any means like solid earthing, or impedance earthing etc. due to that if any earth fault occur in any phase the other phase voltage increase root 3 times from the earth but faulty phase voltage become zero from the earth. But your line voltage remains the same.
It is prevented by use of NGR
Floating neutral means due to if voltage in all three phase is not same due to asymmetrical load at output then voltage drop through all phase are not same so neutral point is shifted from center is prevent by maintain constant load at all phase
Floating neutral means ungrounded neutral
#What is Skin effect in Cable?
Whenever an alternating current pass through the conductor the current density is near the surface of the conductor is greater than the core (middle of the conductor).
#What is cogging in motors?
Cogging phenomenon is seen in the case of 3 phase Induction Motor (SCIM-Squirrel Cage Induction Motor).
When number of stator and rotor slots are exactly equal and polarity of conductors facing each other in slots is opposite, In such cases, there is a magnetic locking between facing conductors housed in stator and rotor slots. Since the rotor inertia is large so motor does not start at all. This is called Cogging.
This is avoided by skewing the rotor conductor slots over the surface by angle in the range of 5 degree.
#What is relation between voltage and current and don't take resistance here. If we suppose say when there is increase in voltage what happens to current? Is it decrease or increase?
It is further added to my answer that if a fixed amount of power is to be transmitted, higher the voltage, lower will be the current.
For a particular power to be transmitted then for increase in voltage we can see decrement in current....this principle is generally used in transmission lines....
#What is Modulation and need for Modulation?
Modulation is a scheme which alters some characteristics of the high frequency carrier in accordance with the low frequency message signal called the modulation signal.
There are three types of modulation:
1. Amplitude modulation (AM): In this type of modulation frequency and phase remains same, only the amplitude changes.
2. Frequency modulation (FM): In FM the peak amplitude and phase remains constant but as the amplitude of the information changes, the frequency of the carrier changes correspondingly.
3. Phase Modulation (PM): PM is used in the same system as an alternative to frequency modulation. In phase modulation, the phase of the carrier signal is modulated to follow the changing voltage level of the modulating signal.
Need for Modulation:
1. Efficient Radiation
2. Frequency Translation
3. Multiplexing
4. Reduction of Noise
#Why transformer neutral earthing? What are advantages?
Transformer neutral in distribution transformer is earthed because:
1) Keep neutral at zero potential,
2) keep all three phase voltage balance even if load is unbalance,
3) To provide return path for current in case of earth fault condition,
Neutral is grounded in transformer
1) Is for restricted earth fault protection (REF)
2) To eliminate the arching ground fault
#Why Flexi Links are used in between generator terminal output & bus duct conductor?
In output of Generator terminal and Bus duct, we use flexi links i.e. no hard connection because to avoid the transfer of vibration of generator to bus duct.
That has also something to do with avoiding high tension on the generator terminals due to the physical weight of the hard bus bar. Plus, limiting the transfer of the generator vibration to the bus.
#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 inductance will not act; only resistance will act in the circuit. So high current will flow through primary side of the transformer. For this reason coil and insulation will burn out.
Mathematical reason is that net impedance of a system is formed of
Z = sq.rt(X.sq + R.sq)
here X = 2*pi*f*l since f = 0 in DC,
X also becomes 0.
Hence Z also =R which is very small in this case. When voltage applied will be like short circuit hence the winding will burn.
#What is various impedance in any machine?
In a synchronous machine, there are three kinds of impedances
1. Xd - synchronous reactance; (used in steady state modeling) - a special case is salient pole machines where sync impedance is split into direct axis impedance and quadrature axis impedance , i.e. Xd and Xq . See double reactance theory for more details
2. Xd' - transient reactance ;( used in transient modeling)
3. Xd'' - subtransient reactance.
All other machines have three kinds of impedences which are used in power system modeling
1. positive sequence impedence
2. negative sequence impedence
3. zero sequence impedence
#How do you convert the lt amps on a transformer to the ht amps?
The formula is given below by using which u can find out the secondary current
V₁I₁(FOR LT) = V₂I₂(FOR HT)
I₂= (V₁I₁)/V₂
#What is the meaning of harmonic in AC waves and when which are produce and what is the effects on line and give any other information about harmonics
The phenomenon of generation of signal which have multiple times of frequency of supply signal is known as Harmonics of that applied signal....
Normally harmonics may generate in different way....
It affects the power quality of electric signal on line...
#How feedback transformer works to feedback power in Slip Power Recovery System?
Feed Back transformer is a just Step up transformer.
it voltage up, up to system voltage.
#I have one transformer 1000kva, 11/.433kv. we are using 200/5 amp. ct unit for measuring purpose, & one 800 amp. V.C.B Unit for the same t/f operate, I wants to known the primary current of T/F is only 53 amp. why we used 800 amp. V.C.B ? & 200/5 amp. CT Unit.
You are right, you should fight with your electricity board regarding the HT Metering CT ratio. You should actually required 100/5 ratio. But while deasigning VCB it is always prfarable to go on higher side because it can sustain the heavy short circuit current. One thing should be noticed that the protection CTs should be as per the Primary current of the ransformer.
#What is the full form of MCCB
Molded Case Circuit Breaker
#Is it possible to increasing the real power generation by reducing the reactive power in power generation plants?
When we bring down the reactive power, there will be some decrease in the load current of the machine. Hence we get more allowance to get additional load on it which eventually results in more power output from the machine.
#What is the need of motor starter?
In almost all the motors, in starting condition, Armature current will be very high. To control that current, starters will be used.
· To control the starting current of the motor thus avoiding overheating of the windings.
· To avoid under voltage hence tripping of other equipments on the same line i.e. for big motors
· Able to vary the speed of the motor .Provides different starting/stopping points of the same motor.
· Provides control measures of over current, earth fault i.e. overload relays, circuit breakers, fuses
· Provides power isolation mechanism when one works on the motor i.e. fuses, breakers, isolators
· Able to know the current taken by the motor i.e. ammeters
#How is working the auto transformer?
If you don’t get confused with the name (auto-transformer) and its internal construction, it is same as normal transformer as far as its working is concerned.
The construction is little different. Primary and secondary windings are physically same i.e. only one winding is there. This winding has multiple taps. The winding between common tap and one tap is Primary winding. The winding between common tap and some other tap is Secondary winding.
#What is difference between A.C & D.C Resistance?
d.c resistance is only resistance due to the virtue of
conductor property only, a.c resistance is due to both
virtue of conductor property and skin effect.
#Describe the difference between a microcontroller and a general purpose computer.
Microcontrollers are nothing but regulators based on microprocessors. Microcontrollers are used for certain predefined operations like in robotics, temperature sensor devices etc.
Computers are used to design microcontrollers.
Micro controllers are low level controllers designed for
specific functions. on the other hand the general purpose
computer has a high level computing compared to micro
controllers.
Micro controllers have small memory like RAM ROM compared
to GP comuters
Also the type of OS porting varies in both the controllers
#What is the relation of current with torque when motor started in Y-connection???
Both current & torque are high in star connection.
#IN 3 phase induction motor where we give i/p supply & how its working?
The input supply is given to the 3 phase stator windings. Due to this the rotating magnetic field is created. This rotating magnetic field cuts the rotor bars. Due to this emf is developed in the rotor bars according to Faraday's law. This emf causes current to flow in rotor bars which interacts with rotating magnetic field of stator. Hence force is experienced
by rotor and it rotates.
#What is battery Amphere-hour? How the battery back up time period is calculated?
The battery is rated in ampere-hour. It means either "X" NOS OF AMPERE CURRENT WILL BE DELIVERED FOR "Y" NUMBER OF HOURS BEFORE IT GETS DISCHARGED , so the ampere-hours will be XxY.
The back up time will depend on the load applied to the battery in terms of watts/kws ....... amp-hrs divided by watts
Ampere Hour=(Watts/DC Voltage)*(1/Inv.Efficiency)*
(1/Batt.Efficiency)*No.of Hours
Ex:W=500W V=12V Inv eff.=0.9 Batt.eff=0.7 Time=1hour
Then Ah=(500/12)*(1/0.9)*(1/0.7)*1
=66.13
It means for 1 hour backup we can use 65Ah Battery for 500W/12V Inverter.
Similarly,for 2 hours Ah=65*2=130Ah
#What is the formula to calculate capacitor bank rating for a certain load?
Calculation of Required Kvar to Raise Power Factor
A negative power factor is interpreted as a leading power factor. A positive power factor is considered a lagging power factor. A negative kvar value should be interpreted as capacitive kvar that shall be removed. A positive kvar value should be interpreted as capacitive kvar that shall be added.
Calculation of Required Kvar
Known Variables: Initial or Present Power Factor, Desired Power Factor, and Real Power of Load
KVAR = KW (Tan θ_i- Tan θ_d)
Where,
θ_i = cos^-1 Pf_i = Initial Power Factor Angle
θ_d = cos^-1 Pf_d = Finial Power Factor Angle
Top of Form
Bottom of Form
#What is AVR in Generators and How is it working.

AVR MEANS AUTOMATIC VOLTAGE REGULATOR.AVR IS HAVING TWO INPUT TERMINALS AND TWO OUT PUT TERMINALS OUT PUT IS GIVING DC VOLTAGE .IT IS CLOSED LOOP CONTROL SYSTEM. I WILL GIVE ONE EXAMPLE . AVR HAVING TERMINALS LIKE S1, S2, AND F1, F2. S1 AND S2 ARE INPUT TERMINALS THAT IS CONNECTED TO OUT PUT OF THE ALTERNATOR VOLTAGE. F1 AN F2 IS CONNEDCTED TO FIELD OF THE EXITER TERMINALS. I WILL EXPLAIN HOW IT IS
WORKS. EXITER IS COUPLED WITH ALTERNATOR ROTOR. EXITER FIELD IS HAVING OWN RESIDUAL MAGNITISUM DUE THIS MAGNETISUM ALTERNATOR GETS SMALL VOLTAGE THAT IS AROUND 10-20VOLTS AT OUT PUT VOLTAGE. U WANT SEE THIS VOLTAGE REMOVE F1 AND F2 CONNETIONS CHECK VOLTAGE AT OUT PUT TERMINALS. THESE OUT VOLTAGE IS SENDING TO AVR INPUT TERMINALS THAT IS S1 & S2 THEN AVR GIVES SAME VOLTAGE TO EXITER FIELD THEN EXITER FIELD GIVES MORE MAGNETISUM DUE TO THIS ALTERNATOR GIVES MORE VOLTAGE AGAIN THIS VOLTAGE GIVEN TO AVR LIKE THIS THESE CYCLES GOIES ON UP REACH REQUIRED VOLTAGE OF THE ALTERNATOR. AT REACHING OUT PUT REQUIRED VOLTAGE AVR IS NOT SENDING MORE VOLTAGE TO EXITER THERE IS CUTT OF SET POINT . WHOLE THESE CYCLE TAKEN VERY LESS TIME MEANS MAY BE 1 SEC. AVR OUTPUT VOLTAGE IS DEPENDS UPON THE COMPANY OF THE AVR AND REQUIREMTNT OF THE ALTERNATOR.
AVR is heart of genset .its developing & maintain the genset voltage as constant .its building voltage by reference of voltage produce by genset .for Ex: if genset rated voltage is 11kv means intially genset produce some minimum voltage (7- 14v)ac only is called resuidual voltage .that voltage is input of AVR .avr developing the voltage upto rated Kv by some programmable circuit in AVr .that O/p voltage is DC only its called as Excitation voltage. we fed the excitation voltage to Excitor in genset.after genset reached rated KV the AVr continusly maintain the voltage as constant
#What happens when the field of an alternator connected to infinite bus is suddenly removed?
if suddenly remove field from generator it will take reactive power from infinite bus ,but it will continuously supplies active power ,that means it act as induction Generator.
#What should be the voltage between neutral and earth of a UPS?
ideally u can says 0 volts, but practically it is not possible to attain 0V due to soil or environmental conditions. it might be as low as possible near about 1-2V Max. Otherwise put water ,check earthling wire....etc.
#what is the MNA in case of d.c. machin? what is the importance of it? how it affect the brush position?
MNA stands for Magnetic neutral axis. It is an imaginary line perpendicular to magnetic lines and passing exactly at the centre of magnetic field. The emf induced at MNA will be zero as the conductors will not cut any flux at MNA. At MNA the conductors will be parallel to magnetic lines. Hence brushes are located at MNA to collect current. During armature reaction the MNA shifts in the direction of rotation of generator. So the brushes should be shifted to new position of MNA to avoid sparking.
#If we convert watt into KVA, which one formula use by us----
watt/p.f=kVA*1000
kW=kVA*p.f
#Where r use in Dc motor in power plant?
we use the DC motors for the fuel handling system and for ID and PD fans.
#why neutral wire don't shock?
To experience a shock,there must be a potential difference between you & the wire you are going to touch it.As neutral is always at zero potential,it doesn't shock you when you touch it.
#what is high pot test on 11kv switchgear and how to apply high voltage on panel what is spheare gap and how to determine its celeberation.
A sphere gap is a type of voltmeter. It contains 2 spheres one of it is connected to Earth n second is connected to the point of which voltage is to be measured. The gap between 2 spheres is reduced till a breakdown takes place. After a break down the gap is measured n the gap is compared with a standard table. Eg. If B/D takes place at 2mm then the voltage recorded in standard table for 2mm is known.
#what is international rules of electrical?
the international rules of electricity is the rule that offer all nations to follow the stndard process to generate, transmit, distribute, and utilize the electriciy at the standared parameter(rated generating voltage, frequency, load power factor, load current, synchronous speed etc)standardize by ISO
#why transformer does not obey ohm law?
Ohm's law applies to electrical circuits; it states that the current through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is: I = {V} / {R},
where I is the current in amperes, V is the potential difference in volts, and R is a circuit parameter called the resistance (measured in ohms, also equivalent to volts per ampere).
It is clear that the Ohms law is applicable in case of purely resistive loads (R) or linear loads. The transformer constructed with windings on primary and secondary sides are inductive in nature which are also non-linear. Hence Ohms law is not applicable to transformers.
#to improve the existing power factor whether capacitors have to be added in series or parallel.and why
The capacitor must be connected in parallel to the load. The capacitive reactance value will be add in parallel. The Power factor is depends up the capacitive reactance. To give lead power factor the value of capacitance should be more. For the reason we add capacitance connecting parallel
#WHAT HAPPEN WHEN CT SECONDARY OPEN DURING LOAD?
If the CT secondary open during load than the voltage across the secondary reaches to a high value i.e some kilowatts and the secondary current becomes zero hence reduced back emf. The working flux increases and core gets saturated. The secondary e.m.f increases due to increased flux.
Due to the above reason the primary gets overheated and core also gets heated and its dangerous for person working on secondary.
#What is stray current in DC Traction?
Current flowing in the soil or water environment in the vicinity of the rail lines and arising mainly from cathodic protection, electric power or traction installations, and which can pass from the environment into the structure, pipelines or other metallic services and vice versa.
#what is the different between earth leakage relay and earth fault relay?
earth leakage relay : monitirs the leakage current continously and ir connected in earthing wire
Earth fault relay oprates on earth fault. It detects earth fault whenever there is short in between phase and earth
#How would you supply a single house hold load from a 3-phase supply coming out of a sub-station?
We can add a neutral line with 3 phase supply (4 wire system), then supply either single or three phase.
#Why capacitor bank does not require in DG Set?
Basically the alternators are designed for 0.8 pF only.Accrodingly the excitor voltage and current is designed. You can not adjust the excitor voltage and current to meet the power factor of load.This will overload the alternator.
#how can testing using megger?
Megger is used to test the resistance/insulation capacity between two conductors be it
(*) Phase and neutral
(*) Line to ground.
(*) Phase to phase
For testing using megger
(*) Connect one terminal to the phase and other terminal to
neutral/ground for testing line to ground resistance.
(*) Connect one terminal to phase and other terminal to
another phase in case of phase to phase resistance.
All the nominal value of resistance must be greater than at least 1Mohm.
#How will you calaculate the Ct ratio based on connected load 500KVA,and 11KV supply ?
p=root3*v*i
500K=root3*11K*I
hence we get
I=62.096
So this is the maximum current flowing to the particular load so we can use 75/1A ratio CT
#What is the difference between reverse power trip & inadvertant trip?
The reverse power relay is used to monitor the direction of power from A.C generators, if the current in the system being monitored is reversed, to a value greater than the customer adjustable preset limit the relay will energise. The adjustable trip point is 2% to 20% of input current. An adjustable time delay of 0 to 20 seconds is provided. Correct setting of the trip point and time delay, will ensure protection against motoring action of the Syn gen.
An inadverdent relay is used to trip the circuit when a syn gen has been energised or excited inadvertently.
#Copper vs. Silver Wire Conductivity
Conductivity

Electrical conductivity is the measure of how well electric current flows through a material. It's measured in units of siemens per meter.

Copper Conductivity

The conductivity of annealed copper is 59 x 10^6 siemens/meter. The resistance of a 24-gauge copper wire 1,000 feet long at room temperature will be about 26 ohms.

Silver Conductivity

The conductivity of silver is about 7 percent higher than for copper, or 63 x 10^6 siemens/meter. A silver 24-gauge, 1,000-foot-long wire would measure about 24 ohms.

Copper Uses

Because its conductivity is the second highest of any metal and its cost is low, copper sees use in most wire, connectors, printed circuit foils and related electrical parts.

Silver Uses

Silver's higher conductivity and cost make it a niche product. It's used as wire and solder in specialty audio electronics.

#Load factor
Load factor (electrical), the average power divided by the peak power over a period of time
#Thyristor Applications
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. The thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor conducts, shorting the power supply output to ground (and in general blowing an upstream fuse).
The first large scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s. The stabilized high voltage DC supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the output DC supply as well fluctuations on the input AC supply. They proved to be unpopular with the AC grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected DC back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade.
Thyristors have been used for decades as lighting dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats. They have also been used in photography as a critical part of flashes (strobes).
Snubber circuits
Because thyristors can be triggered on by a high rate of rise of off-state voltage, in many applications this is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode terminals in order to limit the dV/dt (i.e., rate of change of voltage versus time).
1^st electric lamp in Dhaka 7/12/1901
Bd elect gen capacity 5560mw
Bd elect requirement 6066mw
2008-2009 per head daily production of electcty 172 KWH
2008-2009 bd production 4130 mw
Kaptai hydro elect stablished 1962
Ruppur nuclear 1961
Reb 1977
Khulna bargmount 110mw
Bijoyer alo from Labuan iland of Malaysia 3/8/1999 –130mw
Bd system loss-6.62%
DESA system loss 14.26%
Meghna ghat elect. 450mw largest in bd
1^st gas elect horipur, Shilet
1^st coal elect bor pukuria, dinaj pur
New 4 elect- 1. Narayan gonj shiddirgonj 210mw 2. Shylhet shahji bajar 60mw 3. Gazipur tongi 80mw 4. Dinajpur borpukuria 250 mw
#Windage
Windage is a force created on an object by friction when there is relative movement between air and the object.
There are two possible causes of windage:

the object is moving and being slowed by resistance from the air
a wind is blowing producing a force on the object

#compact fluorescent lamp (CFL)
A compact fluorescent lamp (CFL), also known as a compact fluorescent light or energy saving light (or less commonly as a compact fluorescent tube [CFT]), is a type of fluorescent lamp. Many CFLs are designed to replace an incandescent lamp and can fit into most existing light fixtures formerly used for incandescents.

Compared to general service incandescent lamps giving the same amount of visible light, CFLs use less power and have a longer rated life. Like all fluorescent lamps, CFLs contain mercury, which complicates their disposal.
CFLs radiate a different light spectrum from that of incandescent lamps. Improved phosphor formulations have improved the subjective color of the light emitted by CFLs such that some sources rate the best 'soft white' CFLs as subjectively similar in color to standard incandescent lamps.^[3]

Construction
There are two types of CFLs: integrated and non-integrated lamps.
Parts
There are two main parts in a CFL: the gas-filled tube (also called bulb or burner) and the magnetic or electronic ballast. An electrical current from the ballast flows through the gas (mercury vapour), causing it to emit ultraviolet light. The ultraviolet light then excites a phosphor coating on the inside of the tube. This coating emits visible light.
An electronic ballast and permanently attached lamp tube in an integrated compact fluorescent lamp.
Electronic ballasts contain a small circuit board with rectifiers, a filter capacitor and usually two switching transistors connected as a high-frequency resonant series DC to AC inverter. The resulting high frequency, around 40 kHz or higher, is applied to the lamp tube. Since the resonant converter tends to stabilize lamp current (and light produced) over a range of input voltages, standard CFLs do not respond well in dimming applications and special lamps are required for dimming service. CFLs that flicker when they start have magnetic ballasts; CFLs with electronic ballasts are now much more common.
#What is corona effect?

A voilet glow, hissing noise and the production of ozone gas on the condustor is called CORONA.It generally occur at high voltage overhead transmission lines, when the dielectric voltage becomes more than 30v/cm and the distance between two conductors is greater then their dia, the surrounded air becomes ionized meaning conductive and hissing noise issued due to the saturated air by electronic, and a violet color could be seen. their are some advantages of this phenomena we can get, one of theme in cold region while the ice accumulate over the T.L the heat that emitted due to the accident of electronic dissolve the ice otherwise may the WEIGHT OF T.L becomes so heavy and break the structures of the O.H.T.L.
#Band pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit (a resistor–inductor–capacitor circuit). These filters can also be created by combining a low-pass filter with a high-pass filter.

#Faraday’s law of electromagnetic induction
The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
HVDC electricity transmission
Since modern thyristors can switch power on the scale of megawatts, thyristor valves have become the heart of high-voltage direct current (HVDC) conversion either to or from alternating current. In the realm of this and other very high power applications, both electronically switched (ETT) and light switched (LTT) thyristors^[4] are still the primary choice. The valves are arranged in stacks usually suspended from the ceiling of a transmission building called a valve hall. Thyristors are arranged into a Graetz bridge circuit and to avoid harmonics are connected in series to form a 12 pulse converter. Each thyristor is cooled with deionized water, and the entire arrangement becomes one of multiple identical modules forming a layer in a multilayer valve stack called a quadruple valve. Three such stacks are typically hung from the ceiling of the valve building of a long distance transmission facility.
#Transposition of Transmission Lines
Transposition of Transmission Lines is the transfer of location of a phase conductor in a transmission line pole from left to center lower rack and the other lines from right to left or from center to right. This is done regularly in long transmission lines as a means of balancing transmission line frequency impedance and admittance in a long line. This is usually done in double circuit AC transmission lines
#Vector group
A Vector group is the International Electrotechnical Commission (IEC) method of categorizing the primary and secondary winding configurations of three-phase transformers. Within a polyphase system power transformer it indicates the windings configurations and the difference in phase angle between them.
The phase windings of a polyphase transformer can be connected together internally in different configurations, depending on what characteristics are needed from the transformer. For example, in a three-phase power system, it may be necessary to connect a three-wire system to a four-wire system, or vice versa. Because of this, transformers are manufactured with a variety of winding configurations to meet these requirements.
Different combinations of winding connections will result in different phase angles between the voltages on the windings. This limits the types of transformers that can be connected between two systems, because mismatching phase angles can result in circulating current and other system disturbances.
Symbol designation
The vector group provides a simple way of indicating how the internal connections of a particular transformer are arranged. In the system adopted by the IEC, the vector group is indicated by a code consisting of two or three letters, followed by one or two digits. The letters indicate the winding configuration as follows:
D: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings form a triangular configuration with the terminals on the points of the triangle.
Y: Wye winding, also called a star winding. Each phase terminal connects to one end of a winding, and the other end of each winding connects to the other two at a central point, so that the configuration resembles a capital letter Y. The central point may or may not be connected outside of the transformer.
Z: Zigzag winding, or interconnected star winding. Basically similar to a star winding, but the windings are arranged so that the three legs are "bent" when the phase diagram is drawn. Zigzag-wound transformers have special characteristics and are not commonly used where these characteristics are not needed.
III: Independent windings. The three windings are not interconnected inside the transformer at all, and must be connected externally.
In the IEC vector group code, each letter stands for one set of windings. The HV winding is designated with a capital letter, followed by medium or low voltage windings designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.
The point of confusion is in how to use this notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as 'dY11', but 'Yd11'. The 11 indicates the LV winding leads the HV by 30 degrees.
Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.
Vector group describes the phase shift between primary and secondary windings. It is important that any transformers connected in parallel are of the same vector group which is why it is put on the transformer nameplate.
A convention has been adopted of using the numbers of a clock face to show vector group i.e. A transformer DY1 has a phase shift of 30 degrees (The D and Y describe the winding type, delta or star).
Problems on Machine
Parallel operation of transformer (single-phase)
Ex.1: Two 1-phase transformers with equal turns have impedance of (0.5 + j3) ohm and (0.6 + j10) ohm with respect to the secondary. If they operate in parallel, determine how they will share a total load of 100 kW at p.f 0.8 lagging?

to be continued…….

1 comment:

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