Suggested Answer Power

Dec 2009

1 a i High starting torque ii Low starting current

b)

i Percentage slip

At standstill, the synchronous speed and the rotor speed are the same% slip is 100%

ii Torque produced at synchronous speed =

c) Motor output=30KW; Motor speed = 0.9 synchronous speed ; Rotational losses = 1.4KW

i Rotor copper loss

%slip

Rotor copper loss

ii Operating efficiency

2 a

Armature reaction is due to the magnetic flux set-up by the armature current which distorts the main flux distribution as show above

b)

i) Sparking at the commutator contactii) Wearing of carbon brush Resulting in increase in motor speed and reduction in generator terminal voltage

c)

i Compensating winding ( interpoles ) ; A set of compensating windings is introduced or slotted in the field winding arrangement to compensate for the loss due the distortion caused by the armature reaction on the flux distribution.

ii Moving Brush Gear ; The movable or adjustable brush gear is used for adjusting the brush contacts for proper and firm connection with the commutator to avoid sparking.

3 a

With the KVA Value stays the same, the power in KW @ .094 lagging

The additional power that the transformer can generate @ 0.94 lagging = 849.03- 560= 289.03kw

b)

Cos ;

The horizontal component of load current (I) @ 0.7 pf lagging

The vertical component of load current (I) @ 0.7 pf lagging

The horizontal component of load current ( ) @improved pf of 0.96lagging

Since the horizontal component of load current before improvement is equal to the horizontal component of load current after improvement. It implies that;

The new supply current= 49.6A

(ii) KVAR rating of the capacitor required fro this improvement is given by

OR

Vertical component of the load current (I after improvement

The capacitor current required to be injected to achieve the improvement is given by

For single- phase,

The KVAR rating

c)

Methods of power factor improvement

i) Using of static capacitor for individual loadii) Using of synchronous motor for overall load

4 a

b)

let

Power factor is the cosine of the phase angle= cos42 = 0.74

When the connection to the current coil of wattmeter is reversed, it gives a negative reading. Thus,

5 a

i Power output in kw @ unity power factor @ half-load=

The copper loss

Efficiency @ unity power factor @ half-load

ii Power output in kw @ half-load @ 0.65 power factor

Copper loss remain the same since the load is still half.( i.e 229w )

Efficiency @ 0.65 power factor @ half-load

b)Efficiency of the transformer is maximum when the variable losses is equal to constant losses i.e copper loss = iron loss = 615w

c)

6 a

In an IT network, the distribution system has no connection to the earth at all, or it has only high impedance connection. In such systems an insulation monitoring device is used to monitor the impedance.

Exposed conductive part- This is the metal work of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live, but which may become live under fault condition.

c)

Advantages of mineral insulated, metal sheathed cables compared with pvc insulated and sheathed cables

i It is mechanically robust and resistance to impactii It is water proof and resistance to ultraviolet light and many corrosive elementsiii It does not initiate an explosion even during circuit fault conditionsiv It does not contribute fuel or hazardous combustion products to a fire and cannot propagate a fire within a building.

7 a

The term split-phase as its applied to single-pase induction motor is the connection of an auxiliary winding of smaller gauge wire in parallel with the main winding so that the supply current to the motor is shared between these windings for the purpose of creation of rotating magnetic field.

b)

A single- phase induction motors are not self-starting because for a rotating magnetic flux to be produced, two anti-phase current must be involved.In split-phase induction motor the anti-phase currents are produced by connecting an auxiliary winding of smaller gauge in series with reactor both connected in parallel with the main winding as show below.

d)

8 a

ii armature copper loss

iii Output power of the generator (VI) = 12000w

The input power to the generator = Output power of the motor driving it = Output power of the generator + total losses

b)

9 a

b)

Induction relay have inverse- definite minimum time (IDMT) time- current characteristics in which the time varies inversely with current at low fault currents, but attains a constant minimum value at higher currents. This constant minimum value depends upon the adjustments. Further adjustment is possible by means of tapping on the relay winding.

c)

FACTORS

i Where automatic and remote protection system is requiredii Where rapid action to restore supplies following a fault is necessary.

10 a

i )Magnetising component of the no- load current( I

I no-load current= 3A

ii Power component of the no-load current

b)

Primary current

June 2009

b)

Reading of each wattmeterLet w

w

2 a

For open-circuit test, rated voltage is applied at the primary side while the secondary side is open-circuited. The power measured or recorded by the wattmeter is the iron-losses

For short- circuit test, small voltage of the range 10-12V is applied at the primary side while the secondary side is short-circuited, the power measured or recorded by the wattmeter is the copper losses.

b)

Output power of the transformer in KW at 0.8 power factor lagging at full-load

c)

The e.m.f induced in the primary winding of a transformer

The e.m.f induce in the secondary winding of the transformer For an ideal transformer, the power input is equal to the power output

3 a

A single-phase induction motors are not self starting because for a rotating magnetic flux to be produced, two anti- phase currents must be involved. In a single-phase induction motor, the anti-phase currents are produced by the connecting an auxiliary winding in series with a reactor both connected in parallel with the main winding as shown below.

When the motor has come up to about 70% to 75% of synchronous speed, the auxiliary winding may be opened by a centrifugal switch, and the motor will continue to operate as a single-phase motor.

b)

i Method of starting is by connecting an auxiliary winding of smaller-gauge wire in parallel with the main winding.

ii The direction of rotation may be reversed by reversing the connection to the auxiliary winding.

c)

4 a See the attached graph

b)

5 a

i Excessive billii Poor power efficiency

b)

Total kw = 200+ 25 = 225kw

Total kva =

The combined power factor before improvement=

The kvar rating of capacitors required to improve the power factor of the combined load to 0.95 lagging

ii With the capacitors in circuit and the motor load switched off and the lighting and heating load reduced to 10kw

Overall power factor = cos 83.2 = 0.12 leading

6 a

In oil break circuit breaker, the contact are separated under the whole of the oil in the tank. There is no special arc control system other than increasing length caused by separation of contacts. Arc extinction occurs when a critical gap is reached between the contacts.

ii Plain air break circuit breaker cool the gases to naturally deionized them, causing arc interruption. The arc can be stretched. Its resistance can be increase by increasing its length. The increase in resistance is significant so that the current and voltage are brought into phase.

Advantages of oil over air

1 Oil is very good insulator2 Has a high dielectric strength

Disadvantages of oil over air1 There is no special control over arc other than increase in length by separating the moving contacts. And for successful interruption long arc length is required.2 There is risk of fire.

c)

7 a

b)

The types of three- phase load for which the two-wattmeter method of total power measurement is Balanced three-phase loads.

c)

The earth electrode under test is driven to the ground at the same ground level with two steel rods placed at different distance from the earth electrode under test. The distance of the two steel rods from the earth electrode under test is properly chosen to avoid the overlapping of the resistance area . The earth tester is connected as shown above and operated accordingly and the resistance value display by the earth tester is the resistance of the earth electrode under test.

8 a

b)

This type of starter circuit uses an autotransformer to apply reduced voltage across the windings of the motor during start-up .Three autotransformers are connected in the star configuration and taps are selected to provide an adequate starting current for the motor. After a certain time lapse, full voltage is applied to the motor bypassing the autotransformers.

9 a

b)

Voltage drop @ 0.6 power factor =

b) Starter are use with a large d.c motor to protect the armature winding from high current during loading.

c)

June 2008

1 a

With the KVA Value stays the same, the power in KW @ .095 lagging

The additional power that the transformer can generate @ 0.95 lagging = 950- 600= 350kw

b)

Cos ;






(iii) KVAR rating of the capacitor required fro this improvement is given by

c)

The advantage of connecting a static capacitor used for power factor improvement of an individual load as near as possible to the load terminal is that it prevents over correction and voltage surge since power factor vary with load. Being lower at low load.

2 a

3 a

Advantage of alternating current system over direct current system

i It can be step-up or step-down

Disadvantagei It can not be transmitted over the same size line using the same size tower

b)

Separate protective earth (PE) and neutral (N) conductors from supply to consumer, which are not connected together at any point after the building distribution point. That is PE and N are separate conductors that are connected together only near the power source.

C)

The function of an isolator in distribution system is to cut-off supply from all or a discrete section of the installation by separating the installation or section from every source of electrical energy for reasons of safety.d) The main advantage of IDMT relay is that it has time-current characteristics in which the time varies inversely with current at lower fault currents, but attains a constant minimum value at high currents

4 a

Resistors are connected in series with each of the phase winding as shown above when starting three-phase wound-rotor induction motor.

b)

The three features of a three-phase synchronous machine when operating in the motor mode are;i High torqueii constant speediii High efficiency

5 a

Four typical metering requirementsi Energy ii Maximum demandiii KVAiv Power factor

b)

c)

The multiplying factor =

The actual power taken by the load=

6 a i The eddy-current loss in a transformer can be reduced by laminating the iron –coreii Two reasons for copper loss in a transformer are due to primary and secondary windings

b)

7 a

b)

The field current of a shunt generator is a function of the terminal voltage. Increased loading cause a drop in excitation current. The induced voltage of shunt generator is easily controlled by varying the excitation current by means of a reheostat connected in series with the shunt field winding.

c)

Critical field resistance when referring to a d.c generator is a certain value of field resistance at which generation does not possible.

d)

Two forms of loss occurring in a d.c machine are;i constant lossesii variable losses

8 a

i

b)

The starting current is 5 to 7 times of the full-load current of a typical three-phase cage rotor induction motor.

c)

For maximum torque to occur, the rotor resistance must be equal to the rotor reactance@ Standstill

@ Running

d)

9 a

b)Advantages of using relay for system protection

i It detects system failures when they occur and isolate the faulted section from the remaining of the systemii It mitigates the effects of failures after they occur. Minimising risk of fire, danger to personal and other high voltage system.

c)

Fusing Factor- Is the ratio greater than unity of the minimum fusing current to the current rating.

d)

The function of arc chute in circuit breaker is to divide the arc and cool it.

10 a

b)

Advantage of autotransformer over double woundi Cost saving since less copper is needed

Disadvantagei Since the primary and the secondary winding are not electrically separated, an open circuit in the secondary causes a full primary voltage appearing across the secondary.

c)

d)

The reason for an autotransformer having a primary voltage of 230V not suitable for use in circuit requiring a secondary voltage of 12V is because of the possibility of open circuiting of secondary winding which will cause a 230V to be applied across 12V circuit resulting in over voltage.

Dec 2008

1 a

b)iAdvantage of autotransformer over double wound Cost saving since less copper is needed

ii The danger which may arise if an autotransformer is operated with a high voltage transformer ratio is that if the circuit of primary winding is open circuited, it will results to high voltage across the secondary circuit(side ) of the transformer..

c)

d)

Voltage regulation of a transformer is the difference in the terminal voltage of the transformer at no-load and at full-load

b)

c)The reason for one wattmeter giving a reverse reading is that the connection of the current( fixed ) coil of the wattmeter has been reserved.

4 a

Armature reaction occurs in a d.c motor because of the magnetic field set-up by the armature current which distorts the flux distribution of the main flux as show in the diagrams below.

b)

The two methods of overcoming the effect of armature reaction on commutation in a d.c motor are:

i Compensating winding ( interpoles ) ; A set of compensating windings is introduced or slotted in the field winding arrangement to compensate for the loss due the distortion caused by the armature reaction on the flux distribution.

ii Moving Brush Gear ; The movable or adjustable brush gear is used for adjusting the brush contacts for proper and firm connection with the commutator to avoid sparking.

c) The speed of a d.c shunt motor increases if resistance is connected in series with the field winding because, the field current decreases with increase in the field resistance results in reduction of flux produced. The relationship between the speed and flux is inversely proportional. i.e as flux increases, speed reduces .

5 a Uses of Transformer in transmission and distribution systems.

i For step-up and step-down of electrical power, voltage and currentii For coupling circuit of different voltage levels

b)

Three wire three-phase distribution system is use where neutral connection is not needed on the load. Such as delta connected load. The disadvantage here is that single- phase load can not be obtained on this distribution system.

Four wire three-phase distribution system is use where neutal connection is needed on the load. Such as star connected load. The advantage here is that single-phase load can be obtained on this distribution system.

c)

Advantages of copper conductors

i lower resistivity and higher conductivityii copper conductors may be annealed or hard-drawn

Advantages of mineral ( magnesium oxide ) insulation

i It is water proof and resistance to ultraviolet light and many corrosive elementsii It does not initiate an explosion even during circuit fault conditions

6 a i)

ii)

7 a)

ii )

b)

8 a

b) The salient-pole type of rotors are not suitable for high speed machines because they can not withstand the centrifugal forces developed in the large sizes at high speed.

c) Essential differences in the construction between cage rotors and wound rotors for induction motors are:

The cage rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which are not wires but heavy bars of copper , aluminum or alloys. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings , hence , it is not possible to add any external resistance in series with the rotor circuit for starting purposes. The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew. While, the wound rotor is provided with three-phase , double- layer distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound three-phase even when the stator is wound two- phase . The three- phases are starred internally. The other three winding terminals are brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them. These three brushes are further externally connected to three-phase star-connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor.

d) The percentage slip when the rotor of a three-phase machine is at standstill is 100%

9 a

b)

The field current of a shunt generator is a function of the terminal voltage. Increased loading cause a drop in excitation current. The induced voltage of shunt generator is easily controlled by varying the excitation current by means of a reheostat connected in series with the shunt field winding.

ii The method of restoring the terminal voltage of the above generator to its previous value is by connecting a rheostat or variable resistor in series with the field winding as shown below.

10 a

TNC-S Supply system

In this system of earthing, the neutral and the earth ( protective earth conductor ) are combined at the source but separated at the consumer end.

b)

The purpose of earthing system are :i To protect the personnel from severe electric shockii To protect electrical appliances from damage

c)

The advantage of a closed ring main system of distribution compared with a radial system is that it is more reliable in terms of provision of constant supply of electricity to the final circuit.

d) The function of circuit breaker is to automatically and quickly break electrical circuit during fault condition.

June 2007

1 a

b)

Power factor is the cosine of the phase angle= cos42 = 0.74

When the connection to the current coil of wattmeter is reversed, it gives a negative reading. Thus,

2a

ii

b)

i

ii

3 a

b) The use of direct-on-line starting is sometimes limited in terms of the power rating of the motor because of the high starting current associated with induction motor in which the supply voltage to the motor need to be reduced at starting in order not to affect the operation of other appliances in the same circuit. The direct-on-line starting lack the provision for voltage reduction at starting thus, limited to small induction motor of fractional horse- power.

4 a

The e.m.f induced in the primary winding of a transformer

The e.m.f induce in the secondary winding of the transformer For an ideal transformer, the power input is equal to the power output

b) The core construction of a shell-type transformer in which the primary and secondary windings are wound on the centre limb of the transformer core.

c)

The main advantage of the shell-type of construction compared with the core-type is that the leakage flux is reduced.

5 a

The power factor of a single-phase circuit may be determined by connecting voltmeter across the circuit, ammeter in series and the wattmeter as shown in the circuit above.

b)

Cos ;






KVAR rating of the capacitor required fro this improvement is given by

OR

Vertical component of the load current (I after improvement

The capacitor current required to be injected to achieve the improvement is given by

For single- phase,

The KVAR rating

c)

i Suitable method of power factor improvement for individual loads is the use of shunt capacitor connected across the loadii Suitable method of power factor improvement for overall systems is by the use of synchronous motor.

6 a In an IT supply system of earthing , the distribution system has no connection to earth at all, or it has only high impedance connection . In such system , an insulation monitoring device is used to monitor the impedance.

b)

Exposed-conductive-part is the metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live , but which may become live under fault conditions.

c) Four advantages of mineral-insulated, metal sheathed

i It is mechanically robust and resistance to impactii It is water proof and resistance to ultraviolet light and many corrosive elementsiii It does not initiate an explosion even during circuit fault conditionsiv It does not contribute fuel or hazardous combustion products to a fire and cannot propagate a fire within a building

7 a

i

9 a

b)

Earth fault loop impedance is the impedance of the line-earth loop path and neutral –earth loop path during fault or under fault condition.

c)

Resistance area is the area where voltage gradients exist.

ii The reason for considering resistance area when measuring earth electrode resistance is to prevent overlapping of the resistance area of the actual electrode and the auxiliary earth electrode . 10 a

b) 8% of 12.75=1.02AThe new armature current= 12.75-1.02 =11.73 A

Note: it is assumed that the field excitation is kept constant

Dec 2007

1 a

ii)

b) When resistance is connected in series with fielding winding, the field current is reduced, reducing the flux produced. Since the flux is inversely proportional to the speed. The speed of the

motor increases. i. e

c) The two signs of poor commutation , resulting from the effect of armature reaction that may be visible in d.c machine are;

i) Sparking at the commutator contactii) Wearing of carbon brush Resulting in increase in motor speed and reduction in generator terminal voltage

2 a Three disadvantages of low power factor CONSUMER

i) Extra-cost of power consumptionii) Poor power efficiencyiii) High installation cost

SUPPLIERi) Excessive power loss due to reactive powerii) Poor voltage regulationiii) High cost of installation

b)

Cos ;The horizontal component of load current (I) @ 0.71 pf lagging The vertical component of load current (I) @ 0.71 pf lagging

The horizontal component of load current ( ) @improved pf of 0.95lagging Since the horizontal component of load current before improvement is equal to the horizontal component of load current after improvement. It implies that;


KVAR rating of the capacitor required fro this improvement is given by

=

3 a i) Dielectric stress is the stress placed upon a material when voltage is applied across it. ii) Dielectric strength is the material’s ability to withstand voltage breakdown. It is the insulation ability to contain or withstand voltage without breaking down.

b) One method by which a more uniform distribution of dielectric stress may be achieved in high voltage cable is by inter-sheathing.

Four environmental conditions which may influence the choice of cable for particular applications are :i) Tempatureii) Chemicaliii) Undergroundiv) Water or oild) Two advantages of the materials used in PVC-Insulated , PVC Sheath are :

i) For insulationii) For mechanical protection

4 a

i) Purpose of measuring the insulation resistance of a wiring system is to help ensure specifications are met and to verify proper hook-up and prevent electric shock.

iii) Two dangers which could develop if a wiring system has an unacceptable low value of insulation resistance are; Electric shock and Short circuit

b) The combined insulation conductance

c) Testing of continuity of circuit protective conductors and bonding conductors of an electrical installation is necessary before it is put into service to ensure the safety of the personnel and prevent the electrical appliances from damage incase of faulty condition.

5 a) A synchronous motor is not self- starting because the magnetic flux produced by the stator ran Ahead ( synchronous speed) of the magnetic flux produced by the rotor. For torque to be developed , the magnetic flux of the rotor must be brought to the speed of the magnetic flux produced by the stator.

b) i) Using a prime-mover ( induction motor )ii) Using damper winding

c)

i) Low speed------------------ Salient pole rotorii) High speed------------------- Cylindrical rotor

d)

6 a)

b)

c) Advantage of autotransformer over double-wound transformer

i) Cost saving since less copper is needed

Disadvantage

i) Since the primary and secondary windings are not electrically separated, an open-circuit in the secondary winding causes a full primary voltage to appear across the secondary winding

ii) For starting induction motor and for inter-connecting systems that are operating at approximately the same voltage.

7 a)

A combined protective earth – neutral (PEN ) conductor fulfills the functions of both a protective earth ( PE ) and a neutral ( N) conductor. That the neutral and earth are connected together both at the supply and at the consumer.

b)Exposed-conductive-part is the metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live , but which may become live under fault conditions.

c)Disadvantages of rewirable fuse

i) The fuse element may be replaced with wire of the wrong size either deliberately or by accident.

ii) The circuit cannot be restored quickly since the fuse element requires screw fixing

The main contactor K1 will energize only when the control circuit fuse (F3), backup fuse(F1), and the overload relay (F2) are healthy and the start pushbutton (S1) is pressed.Reduced-voltage configuration (star configuration)Star–delta timer coil (K4) gets power through fuses F3, F1, NC contact of stoppushbutton (S0), and NO contact of start push button. As start PB (S1) is pressed, thetimer coil K4 will pickup and in turn energize the star contactor coil K2. The main linecontactor (K1) coil gets power via the NC contact of S0, NO contact of S1, NO contact ofK2 and remains latched unless the stop pushbutton (S0) is pressed.Now, the main line contactor (K1) and the star contactor (K2) are in a pickup condition,which will drive the motor in the star configuration.Full voltage (delta configuration)As the time duration set on a K4 timer (star to delta timer) elapses, the contactor coil (K3)is picked up and at the same time, the star contactor (K2) is de-energized.Now, the main line contactor (K1) and the delta contactor (K3) are in a pickupcondition, which will drive the motor in a delta configuration. When the motor trips in anoverload condition either in a star or delta configuration, the control circuit alwaysensures that the motor restarts in a star configuration, rather than the delta configuration.

b) For direct-on-line starting (DOL ) The starting current = 5 x full-load

For star- delta starting

The starting current =

For autotransformer starting on 70% tapping

The starting current = 5 x 0.7 x full - current

c)

9 a

Iron-loss is due to eddy current and hysteresis loss. The eddy current loss is due to e.m.f being induced in the winding and core of the transformer. While hysteresis loss is due to molecular

structure of the material from which the core is made from. The iron- loss is not affected by transformer load. It remains constant for a specific transformer. Copper loss is a loss that occurs due o the resistance of the winding or conductor. Copper loss varies with the transformer load by the relationship stated below

b)

c) The main advantage of using a sandwich type of winding in transformer construction is to reduce the leakage fluxes.

10 a

ii

SUPPLEMENTARY QUESTIONS & ANSWERS

DC MACHINES

1 a)

State how the torque developed by a dc motor varies with EACH of the followingi) field excitationii) armature current

b)List FOUR losses occurring in a dc machine.c) Sketch a circuit diagram showing the field connection for a separately excited dc machine

2 a) Show , by means of connection diagrams , two different methods by which a variable resistance may be used to control the speed of a dc shunt –wound motor. In each case state the effect on the speed when the resistance is increased.

SOLUTION

1 a) i) ( Torque is directly proportional to the field current )

ii) ( Torque is directly proportional to the square of the armature current )

b)i) copper lossii) friction and windage lossiii) iron lossiv) brush contact loss

c)

In flux or field control method, the flux can be varied with the help of variable resistance connected in series with field winding. Since the speed varies inversely with the flux, hence, by increase the resistance , the flux reduces and the speed increases.

In armature control method, the voltage across the motor armature is changed with the help of controller resistance connected in series with the armature, an increase in the resistance causes the potential difference across the armature to decrease, thereby decreases the speed. Speed is directly proportional to armature current.

AC MACHINE

1 a)

A 90kw three-phase , six pole induction motor supplied at 415V,50Hz operates on full-load at 0.86 power factor lagging with a slip of 4%. The rotational losses absorb a torque of 16Nm and the stator and rotor copper losses are of equal value. Calculate the i) rotational power lossesii) rotor copper lossv) input powervi) full-load efficiency

SOLUTION

1 a)

2 a)

A three- phase, 550V, 50Hz, 18.65kw, 4 pole induction motor is supplied at the rated voltage and frequency. The starting torque is equal to the full-load torque and at full-load the slip is 4%. Calculate the starting torque in Nm. What will be the approximate starting torque if the supply voltage falls to 520V with the frequency and slip remaining constant.

SOLUTION

b) A 6 pole, three-phase induction motor runs at a speed of 960rev/min when the shaft torque is 136Nm and the frequency 50Hz. Calculate the rotor copper loss if the friction and windage losses are 150w

c) A 50Hz , four-pole , three-phase induction motor operates under a variety of load conditions. Calculate thei) slip when the rotor current frequency is 3Hzii) rotor current frequency when the rotor speed is 750 rev/minvii) slip speed when slip is 4%

SOLUTION

d) Define the following terms when referring to ac induction motorsi) synchronous speedii) slipSOLUTION

Synchronous speed is the speed of rotating magnetic field produced by the stator winding of a three-phase induction motor.

Slip is the fractional difference in speed of synchronous speed and the rotor speed

e) state the type of rotor construction suitable for each of the following synchronous machines

i) low speedii) high speed

SOLUTIONLow speed……………. Salient pole rotor

High speed………………Cylindrical rotor

TRANSFORMER

1 a) A 500 kVA transformer has a full load copper loss of 4kW and an iron loss of 2.5kW. Determine (a) the output kVA at which the efficiency of the transformer is a maximum, and (b) the maximum efficiency, assuming the power factor of the load is 0.75

b) Define the term’ voltage regulation’ of a transformer c) A 500/250V , 10KVA single-phase transformer has primary and secondary winding resistances of

0.3 and 0.02 respectively. The primary and secondary leakage reactances are 1.2 and 0.05respectively. Calculate the

i) equivalent impedance referred to the primaryii) full-load voltage regulation at a power factor of 0.8 lagging

SOLUTION1 a)

Let x = the fraction of the output at which maximum efficiency occur

b)

Voltage regulation is the difference in transformer terminal voltage at no-load and full-load.

c)

2 a) A 15KVA , 1000/ 250V single- phase transformer has primary and secondary winding resistances of 0.8 and 0.1 respectively. The primary and secondary leakage reactances are 1.6 and 0.06Calculate the equivalent

i) resistance referred to the secondaryii) leakage reactance referred to the secondary.

b) Calculate for full-load conditions, thei) voltage drop at a power factor of 0.6 laggingii) per unit regulation at a power factor of 0.95 lagging

TRANSMISSION & DISTRIBUTION

1 a) State two advantages of the insulation and sheathing materials used in each of the following high voltage cables

i) p.v.c insulated, p.v.c sheathedii) paper insulated, metal sheathed

b) Define the meaning of the term ‘dielectric stress’. c) Define the term ‘ fusing factor’

d) state

i) the main effect of excessive dielectric stress in high voltage cables and cable terminations ii) two methods by which the stress may be controlled.

SOLUTION

Advantages of pvc insulated , pvc sheathedi) pvc insulation protection against short-circuit and electric shockii) pvc sheathed provides mechnical protectionAdvantages of paper insulated , metal sheathedi) paper insulation provides insulation against short-circuit and also reduces/ control the dielectric stress on the cable ii) metal sheathed provides mechanical protectionb) Dielectric stress is the stress place on a high voltage cable when a voltage is applied

across it.c) Fusing factor is the ratio of the fusing current to the rating capacity of fuse.d) The effect of excessive dielectric stress in high voltage cables and cable terminations are

insulation breakdown and short-circuit.Methods of controlling stress is by grading and intersheathing

2 a) i) Describe , with the aid of a diagram , the principle and method of earthing of a TT supply system. ii) Describe the main function of an inverse definite minimum time (IDMT) protection relay. b) Define the meaning of the following terms i) earth ii) earth electrode c) state two services where the metalwork shall not be used as a protective earth electrode.

SOLUTION

a)

ii) The main function of an inverse definite minimum time(IDMT) protection relay is for discrimination of fault current.

b) Earth is the general mass of the earthEarth electrode is the electrode that connect the protective conductor to the general mass of the earth

c)

3 a) Show , by a labeled diagram , how a power factor meter is connected into a single- phase installation.

b) Explain why the used of a residual current device (RCD) is required for installations having an increased shock risk.

SOLUTIONa)

b) Unlike earthing system, the RCD trips immediately it senses imbalanced in the current flowing in the neutral and live conductors. This action prevents the personnel from having contact with the fault current.

Suggested Answer Power

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