Section 3 - Electrical Plant Flashcards
Explain leading and lagging on a Sine Wave
Leading is when the wave comes before the reference wave.
Lagging is the wave that comes after the reference wave.
They will always be 120 degrees out so Leading would be 120 and lagging would be 240 degrees.
Define Power Factor?
The factor by which the apparent power is multiplied to give the true power
What 3 factors determine the output of an A.C generator?
- Magnetic field density
- Speed of rotation
- Length of rotor and conductors
Describe a Static Exciter?
The AC generator supplies its own field current via a static exciter unit containing a 3 phase transformer/rectifier to change the AC output to DC. Voltage control is normally by an AVR unit incorporated in the static exciter unit.
List 3 advantages of having a Static Exciter?
- The generator set is shorter due to there being no rotary exciter;
- The static exciter unit can be mounted where convenient, e.g. high on a bulkhead;
- The static exciter contains no moving components so improving reliability.
Describe a Brushless Exciter?
A brushless exciter is an integral part of synchronous generators, serving to provide the necessary direct current (DC) excitation to the generator’s rotor without the use of traditional brushes and slip rings. The system comprises a rotor with separate main and exciter field windings and a stationary exciter stator surrounding the rotor. As the generator rotor rotates, the exciter stator induces an alternating current (AC) voltage. This AC voltage is then rectified to DC using solid-state rectifiers. The rectified DC power is supplied to the exciter field winding on the rotor, generating a magnetic field. This magnetic field, in turn, induces the main field winding, establishing the required magnetic field for power generation. A voltage regulator monitors the generator’s output voltage, adjusting the excitation level by controlling the DC current supplied to the exciter field winding, ensuring precise voltage regulation.
List 2 advantages of having a Brushless Exciter?
- There are no brushes, commutator or slip rings, hence necessary maintenance is greatly reduced and reliability increased.
- The absence of brushes and carbon dust enables closed cycle air cooling systems to be used so further reducing maintenance.
Explain the step by step procedure for Insulation Resistance Testing?
Insulation resistance of an AC alternator, all rotating rectifiers,
the AVR unit, etc. must first be disconnected.
Should only be undertaken by competent persons.
Appropriate Permit to Work
A test voltage higher than the normal operating voltage should be used (Mega tester)
The resistance between each Phase and Earth should exceed 1M OHM.
The resistance of the earth lead on motors and generators should also be routinely tested with an OHM meter and should not exceed 0.1 OHMS.
List typical AC Generator Maintenance
- Insulation testing
- Wiring inspections
- Slip ring cleaning and brush checks (if fitted)
- Tightness of connections and terminals
- Air filter clean / replace
- Anti-condensation heater checks
- Earth bonding arrangements
- Diode check (if brushless)
- Diode failure detection check
What would be the outcome if two generators were attempted to be synchronised out of phase?
Very large physical electro- magnetic torques oppose each other and may cause catastrophic damage to both generators crankshafts, bearings, bedplates, rotors, stators and couplings.
How to adjust the frequency of the incoming generator?
Frequency is directly proportional to the speed of the prime mover (diesel engine). By controlling the engine speed, the frequency can be raised or lowered.
It should come on slightly higher to ensure it will take load and not go into reverse power.
If the voltage is low on the incoming generator, what could be the possible causes?
- The voltmeter is not reading correctly;
- The automatic voltage regulator (AVR) has a malfunction;
- The AVR is not set correctly;
- There is a failure with the exciter.
What are the two required methods for synchronising generators?
- Synchroscope
- Synchronising lamps
Explain step by step procedure for starting, synchronising and paralleling generators?
- Complete generator pre-start checks ( Oil, Coolant, Valves, Fuel).
- Start locally so immediate steps can be taken to shut down if a problem.
- While generator is getting to operating temperature, carryout inspection of generator.
- Once at operating temperature check the voltage of generator, adjust with exciter if needed.
- Check the AVR isn’t in fault.
- Ensure the reverse power and overload trips are reset.
- Check the incoming generators frequency, should be slightly higher than the busbar frequency.
- Switch on check synchroniser.
- Switch on synchroscope.
- When the incoming generator is settled, close the breaker at 10 to 12 on the synchroscope. Ensure the breaker has closed and generator is taking load.
- Balance the load by increasing the speed on the incoming generator and lowering on the busbar generator.
- Balance the load using the KW power meters then check the current ammeters.
- Turn syncroschope and check synchroniser off.
- Carryout periodic checks of the generator and load sharing.
List the types of damaged caused from incorrect synchronising?
- Deformation of stator windings,
- Movement between stator core and frame,
- Failure of rotor diodes on brushless machines,
- Twisted rotor shafts,
- Localised crushing of shaft and keyway,
- Broken couplings.
