LATEST ELECTRICAL TECHNOLOGY MMD ORAL Q & A PART-2

0
1443

Q What are main switch board safety device? Explain about.

Main switch board safety device are

1. Preferential Trip 

2. Over voltage trip

3. Under voltage trip

4. Over current trip

5. Reverse current tip

6. Fuses

7. Automatic circuit breaker

8. Earth lamp

9. Meter

10. Synchroscope

11. Emergency synchronizing lamp

12. Rubber footstep, Insulated hand rail

1. Preferential Trip

❖ Usual setting for overload trip 150% load (50% overload)

❖ When generator load reach 110%, Preferential trip into operation as follow

❖ First trip at 5 sec —shut down non essential load (air condition, accommodation fan, cargo hold fan

❖ Second trip at 10 sec — shut down essential load (Refer plants)

❖ Third trip at 15 sec — shut down main generator as last action, if load still high, due to short circuit or insulation breaking

2. Earth lamp

❖ Set of lamp, show presence of earth fault in distribution system

❖ In a 3-Phase A.C, there have no earth faults, the lamps glow with equal half brilliance.

❖ If an earth fault occurs on one line, the lamp connected to that line dim or extinguished.

❖ The other lamps will glow brighter than before (due to increased voltage).

3. Synchroscope

❖ Synchroscope is instrument

❖ Indicate that voltage, frequency, phase angle of two electrical supply of running machine and incoming machine are in synchronism and parallel

Synchronising method

Synchronscope is main method.

Back up method are

1. Lamp dark method

2. Lamp bright method

3. Sequcnce method

  • In each case, lamps connected between incomer and bus-bars.
  • Sequence method, displays rotation of lamp brightness indicates incomer running fast (clockwise) or slow(anti-clockwise).
  • Incomer is adjusted, lamp sequence rotates slowly clockwise.
  • Circuit breaker closed when top or key lamp is dark and two bottom lamps are equally bright.

4. Reverse current trip

  • Fit in main switchboard to trip and disconnect main circuit breaker, in event of reverse current flow.
  • Prevent generator against running as motor

(a) What are likely consequences of attempting to close incoming breaker when generators are not in synchronism?

(b) How could you monitor the correct instant for synchronizing with and without the aid of a synchroscope?

(c) Make a diagram of connection for a switchboard arranged for controlling two compound generators intended to run in parallel. Explain the sequence of operation for putting a compound generator on to bus bar to which another generator is already connected and for making the incoming generator take a share of the load.

(d) Synchroscope and its connection to main circuit.

(e) Ammeter or voltmeter construction to main circuit.

(f) Earth detecting lamp and its connection to main circuit.

(g) How could you monitor the correct instant for synchronizing without the aid of a synchroscope or synchronizing lamps?

(a)

  • At instant of closing the breaker, voltage phase difference causes a large circulating current between machines which produces a large magnetic force to pull the generator voltages into synchronism.
  • This means rapid acceleration of one rotor and deceleration of other.
  • Large forces physically damage generators and their prime-movers
  • large circulating current trip each generator breaker and have a blackout.

(b) Correct instant for synchronizing without the aid of synchroscope

1. Set of lamp use back-up to the synchro scope

2. Correct synchronized position shown by either of the following methods:

(a) Lamps dark method (b) Lamps bright method (c)Sequence method

3. In each case, lamps connected between incomer and bus-bars.

4. Sequence method, displays rotation of lamp brightness indicates incomer running fast (clockwise) or slow(anti-clockwise).

5. Incomer is adjusted, lamp sequence rotates slowly clockwise.

6. Circuit breaker closed when top or key lamp is dark and two bottom lamps are equally bright.

(b) Correct instant for synchronizing with the aid of synchroscope (or ) (c)Procedures for generators parallel-run

1. To achieve smooth manual synchronizing, incomer’s speed adjusted to obtain approximately same frequency on bus-bar frequency meter e.g. 60 Hz.

2. Incoming generator voltage set by its AVR or manual trimmer

3. Fine tuning of speed observed on synchro scope or synchronizing lamps.

4. Incomer adjusted synchroscope indicator rotates slowly clockwise (fast direction) at about 4 seconds per indicator revolution.

5. Circuit-breaker closed, indicator approaches the 12 o’clock (inphase) position.

6. Pointer of incoming generator’s ammeter very little kick when correctly synchronized.

(c) (d)

(d) Synchroscope

  • Synchroscope has two windings; one connected to bus bars or running machine and other to incoming machine i.e. on the machine side of the appropriate circuit breaker.
  • Where several machines, synchroscope winding, intended to monitor incoming machine, connected to each machine by selector.

(e)

Ammeter

  • Ammeter used to measure current and connected in series with circuit to avoid large voltage drop across ammeter
  • Made very low resistance of few turns of thick wire, consume much power

Voltmeter

  • Voltmeter used to measure voltage and connected in parallel with it.
  • Made very high resistance of thin wire, and consume minimum power.

(f) Earth detecting lamp and its connections to the main circuit

  • Earth indication lamps in a 3 phase A.C. system are arranged as shown in figure.
  • When the system has no fault, the lamps glow with equal half brilliance.
  • If any earth fault occurs on one line, the lamp connected to that line goes dim or extinguished.
  • The other lamps will glow brighter than before due to increased voltage.

(g) Synchronizing without the aid of a synchroscope or synchronizing lamps

  • Synchronized with s500V voltmeter for 440 system
  • Connect pair of voltmeter proves across one phase of incoming circuit breaker
  • Adjust generator speed until voltmeter slowly fluctuate from ‘0’to ‘max’
  • Close circuit breaker when voltmeter indicator pass through zero

Q Synchronous AC generators are usually provided on board ship, Explain in detail one type of generator to

(a) The construction (b) The performance; and (c) The maintenance required.

A. Explain construction and operation principle of synchronous generator(alternator)

B. Explain with aid of simple sketches, basic function of three phase static exciation.

C. Explain aid of simple sketch, basic principle of AC GE and static excitation method of phase compound exciation

(a) (A) AC generator detail construction

  • AC generator have two main parts- stator and rotor.
  • Stator- stator will be armature, generate output voltage.
  • Assemble from laminate steel, 3 sets of phase winding, located slots
  • 6 ends stator coils interconnected and found in stator terminal box.
  • Rotor have two types – (1) salient pole type and (2) cylindrical type.)
  • Salient pole type for marine generators, has projecting poles bolted or keyed onto shaft hub.
  • Field excitation windings are fitted around each pole.
  • Used with medium and low speeds (1800 rpm and below)
  • Cylindrical type rotors are large power, use high speed (1500 ~ 3600 rpm) gas or steam turbine drives. The excitation windings are wedged into axial slots around the steel rotor.
  • Excitation current fed into winding via carbon brushes on pair of shaft mounted slip rings.
  • Brush- piece of carbon, press by springs to make contact with rotating slip ring.
  • Slip ring- Ring for making connection through brushes between winding and external circuit.)

(b)(A)(C) The performance

  • AC generator convert mechanical energy into electrical energy
  • Pairs of electromagnetic poles are driven by the prime mover
  • Flux past fixed coils of wire on stator.
  • EMF induced in each phase, is 120° out of phase with the other two phases.
  • 3 phase winding labeled U-V-W, terminal and bus-bar use R-Y-B

(c) Generator maintenance

1. Shut down prime mover engine and locked off.

2. Main circuit, auto start circuit, heater are switch off, lock out and tag out.

3. Inspect and clean generator rotor and stator windings by electrical solvent.

4. Inspect wire cable damage or insulation.

5. Inspect tightness of terminal connection

6. Check for sign of oil and water contamination

7. Cooling air intake and exhaust opening are not blocked and are free of dirt and dust

(B)(C) Static excitation method

  • Transient voltage response of generator improved by using static excitation method.
  • Static excitation equipment located within generator casing or inside main switchboard.
  • In this method, generator rotor field winding supplied with d.c. current, via shaft mounted slip rings and brush-gear arrangement, from static excitation transformer
  • Rectifier unit which fed directly from generator voltage and current output.
  • This arrangement known as compounding as controlled by voltage (shunt effect) and current (series effect) feedback.
  • On no load, generator excitation is provided by PRI.1 winding of excitation transformer.
  • On load, generator current injects an additional excitation current via PRI.2 of transformer to maintain a constant output voltage.
  • Response times as low as 1.5 second to correct a 15% voltage dip.