Q a) Explain why it is necessary to have reverse power protection for alternators intended for Operation.
b) i. Sketch a reverse power trip.
ii. Explain briefly the principle on which the operation of this power trip is based & how tripping is activated.
Q a) Describe a brush less alternator with a.c. exciter static A.V.R.
State the output voltage characteristics for this type of the machine.
Q a) Give a brief outline of the care maintenance that should be given to the stator and rotor of an A.C. generator.
b) Explain what is likely to occur if the driving power of the one A.C. generator suddenly fails when 2 generators are running in parallel. What safety devices are generally provided for such events?
Q a) State the conditions required prior to the synchronizing of electrical alternators.
b) Describe the type of the cumulative damage that may be caused when alternators are incorrectly synchronized.
c) Explain how the damage referred to in (b) can be avoided or reduced.
d) For 2 alternators operating in parallel state the consequences of:
i. Reduced torque from the prime mover of one machine.
ii. Reduced excitation on one machine.
Q a) Describe the circuit breaker for an a.c. generator using a sketch to show how arcing is controlled.
b) Explain the sequence of events that might occur if the breaker opens on a short circuit & state the check you would require following such event.
c) Give a safe procedure to follow should a main circuit breaker fail to open under the fault condition.
Q a) Sketch a main engine shaft driven generator arrangement with an electronic system for frequency correction.
b) Describe the operation of generator arrangement sketched in (a).
Q Which of the following devices will prevent a D.C. generator from becoming motorized?
a) Over current relay
b) Motorization trip
c) Reverse power relay
d) Reverse current relay
Give a detailed explanation as to why other options were not considered.
Q a) In a.c. generators, voltage dip occurs in two stages.
i. Draw a voltage-time graph showing the pattern of the voltage dip.
ii. Referring to this graph, state with reasons the effect on the electrical system of a small power installation when a large load is suddenly switched on.
b) Explain each of the following categories of voltage control:
i. Error operated
Q a) Sketch a circuit diagram of an emergency generator power supply system suitable for the passenger vessel: including in your diagram the essential services supplied.
b) With reference to an emergency generator state:
i. Its location.
ii. Methods of automatic starting.
iii. The time available for starting & connection of power to switchboard.
iv. Angles of heel & trim up to which it must be capable of operating.
v. The length of time it must be capable of supplying emergency power.
Q a) i. Describe with the help of a sketch the operation of a synchroscope.
ii. State the information obtained from it.
b) Suggest a substitute in the event of synchroscope & stand by light failure.
Q a) Describe with the help of a simple sketch the arrangement of the three phase winding of an alternator showing the neutral point
b) Explain why for the most ships the neutral point is insulated
c) Explain why in some installation the neutral point is earthed.
Q a) Sketch a circuit diagram for an automatic voltage regulator illustrating how the A.V.R. utilizes a Silicon-controlled rectifier to control the excitation system for an alternator.
b) Describe how the A.V.R. monitors output & controls the excitation system.
Q Why upon heavy impact loading self-excited compounded alternator provide the best response in limiting voltage dip & recovery time.
Q a) i. Draw a diagrammatic arrangement of a static or self-excited alternator.
ii. Describe the operation of the self-excited alternator.
b) State why the voltage dip is less in self-excited alternator than in the brushless or conventional alternators.
Q a) List the factors that determine the starting torque of the three phase induction motor. How does this torque usually compared with the value of the rated torque?
b) Draw a simplified circuit of a reverse power relay explain why there is a time delay incorporated before the reverse power relay operates.
c) i. What is direct connected alternator?
ii. How is the direct connected exciter arranged in an alternator?
d) A 12-pole, 3-phase, delta-connected alternator runs at 600 rev/min & supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm & inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine:
i. Frequency of supply
ii. Current in each coil
iii. Current in each phase of the alternator
iv. Total power supplied to the load.
Q a) Describe the effect of the following loads on power factor:
i. Induction motors
iii. Partly loaded motors
iv. Cage type motors.
b) In a 50 kVA, star-connected, 440 V, 3 phase, 50 Hz alternator, the effective armature resistance is 0.25 ohm per phase. The synchronous reactance is 3.2 ohm per phase & leakage reactance is 0.5 ohm phase. Find at rated load & unity power factor:
i. Internal e.m.f Ea.
ii. No load e.m.f Eo
iii. Percentage regulation on the full-load
iv. Value of the synchronous reactance which replaces armature reaction.
Q A 100 kW, 460 V shunt generator was run as a motor on no load at its rated voltage and speed. The total current taken was 9.8 A, including a shunt current of 2.7 A. The resistance of the armature circuit at the normal working temperature was 0.11 Ω. Calculate the efficiencies at:
a) Full load
b) Half load
Q a) Describe the situations in which hand regulation might be desirable for operation of the generators.
b) What are the factors which determine the synchronous speed of the motor?
c) 3-conductors fitted side by side in the stator of a salient-pole alternator. Each generates maximum voltage of 200V (sinusoidal). The angle subtended at the centre of the stator between adjacent conductors is 20 electrical degrees. If the 3 conductors are connected in series, find:
i. r.m.s. value of the effective voltage &
ii. The “breadth factor” using the theory that is the basis of this problem, give one reason why 3-phase current has been introduced
Q a) Sketch a simplified circuit of a reverse power relay explain why there is a time delay incorporated before the reverse power relay operates.
b) List the factors that determine the starting torque of the 3-phase induction motor. How does this torque usually compared with the value of the rated torque?
c) A 12-pole, 3-phase, delta-connected alternator runs at 600 rev/min & supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm & inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine:
i. Frequency of supply
ii. Current in each coil
iii. Current in each phase of the alternator
iv. Total power supplied to the load
Q a) Describe the no load saturation characteristic of a D.C. generator.
b) A 4-pole machine running at the 1500 r.p.m. has an armature with 80 slots & 6 conductors per pole. The flux per pole is 6 x 106 lines. Find the terminal e.m.f. of d.c. generator if the coils are lap connected. If the current per conductor is 100 Amps, find the electrical power.
Q Two shunt generators X and Y work in parallel. Their external characteristics may be assumed to be liner over their normal working range. The terminal voltage of the X falls from 265 V on no load to 230V when delivering 350 A to the bus bars, while the voltage Y falls from 270 V on no load to 240 V when delivering 400 A to the bus bars. Calculate the current which each machines delivers when they share a common load of 500A. What is the bus bar voltage under this condition & power delivered by each machines?
Q a) Explain the purpose of the interpoles & state their magnetic polarity relative to the main poles of both generators & motors.
b) A 200V, long-shunt compound-wound generator has a full-load output of 20 kW. The various resistances are as follows: armature (including brush contact) 0.15 ohm, series field 0.025 ohm, interlope field 0.028 ohm, shunt field (including the field-regulator resistance) 115 ohm. The iron losses at full load are 780 W, and the friction and wind age losses 590 W. Calculate the efficiency at full load.
Q a) Derive an expression for the e.m.f induced in an a.c. generator?
b) A 3000 KVA, 6 pole alternators runs at 1000 r.p.m in parallel with other machines on 3300 V busbars. The synchronous reactance is 25%. Calculate the synchronizing power for one mechanical degree of displacement and the corresponding synchronizing torque.
Q a) What is the effect on the field flux of an alternator current in the synchronous motor that leads the terminals voltage
b) If an alternator supplies the following loads:
i. 200 kW lighting load at utility power factor
ii. 400 kW induction-motor load at 0.8 (lagging), power factor
iii. 200 kW synchronous-motor load, find the power factor of the synchronous-motor load, to give an overall power factor of 0.97 (lagging).
Q a) Differentiate between series and parallel resonances. Draw impedance characteristics of these circuits.
b) A 2,000 kVA, 3-ɸ, 8-pole alternator runs at 750 r.p.m in parallel with other machines on the 6,000 V bus-bars. Find the synchronizing power on full-load 0.8 p.f. lagging per mechanical degree of displacement & the corresponding synchronizing torque. The synchronous reactance is 6 ohm per phase.
Q a) Explain how the efficiency and regulation of a transformer can be assessed by open circuit and short circuit tests?
b) What is meant by equivalent resistance?
Q a) Why is it important to maintain high efficiency of operation and low values of voltages regulation for power transformers?
a) A 100 kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 ohm and 0.01 ohm respectively, and the corresponding leakage reactances are 1.1 ohm & 0.035 ohm respectively. The supply voltage is 2200 V. Calculate:
i. The equivalent impedance referred to primary circuit
ii. The voltage regulation & secondary terminal voltage for the full load having a power factor of
a) 0.8 lagging and
b) 0.8 leading
Q a) Explain the potential hazards if liquid-cooled transformers are used.
b) What are the losses in transformers? Mention the various factors which affect these losses.
c) In a 25 KVA, 3300/233V, single phase transformer, the iron and full load Cu losses are respectively 350 & 400 watts. Calculate the efficiency at half-full load, 0.8 power factor.
Q a) What is leakage flux as it applied to the iron core transformer? How is it taken into account and the analysis of the transformer?
b) The following results were obtained on a 50kVA transformer: open circuit test primary voltage of 3300 V; secondary voltage of 415 V; primary power of 430 W. Short circuit test-primary voltage 124 V primary current 15.3 A, primary power, 525 W, secondary current, full load value. Calculate:
i. The efficiencies at full load & at half load for 0.7 power factor.
ii. The voltage regulations for the power factor 0.7 (i) lagging (ii) leading.
iii. The secondary terminal voltages corresponding to (i) and (ii).
Q a) Show how the power that is transferred across the air gap of the three phase induction motor is represented. Explain the terms. What portion of this is useful power?
b) The primary & secondary winding of a 500 kVA transformer have a resistance of 0.42 & 0.0019 respectively. The primary & secondary voltages are 11000 V and 415 V respectively & the core loss is 2.9 kW, assuming the power factor of the load to be 0.8. Calculate the efficiency on:
i. Full load
ii. Half load
Q a) How does change to frequency affect the operation of the transformer? What makes this ratio different from the ratio of transformer?
b) A 550 kVA, 50 Hz single phase transformer has 1875 & 75 turns in the primary & secondary winding respectively. If the secondary voltage is 220 V, calculate
i. Primary voltage
ii. Primary & secondary currents
iii. Maximum value of flux
Q A three phase transformer has 560 turns on the primary and 42 turns on the secondary. The primary windings are connected to a line voltage of 6.6 kV. Calculate the secondary line voltage when transformer is connected.
a) Star Delta
Q a) Distinguish between power efficiency and all day efficiency. Why is all day efficiency considered more reasonable basis for comparison than ordinary efficiency.
b) The no-load current of a transformer is 5.0 A, power factor 0.25 when supplied at 235 V, 50 Hz. Turns on the primary winding is 200. Calculate:
i. The maximum value of flux in the core
ii. The core loss.
Q A 72 kVA transformer supplies (a) a heating & lighting load of 12 kW at unity power factor (b) a motor load of 70 kVA at 0.766 (lagging) power factor. Calculate the minimum rating of the power factor improvement capacitors which should be connected in the circuit to assure that the transformer does not become overloaded.