Answer MET Question 4
Question: Diesel electric propulsion is now being chosen as the power plant for an increasingly wide variety of vessels.
(a) Sketch a simple layout of such an installation.
(b) Explain the advantages of selecting such a plant.
(b)
In the Figure shown the HV generators form a central power station for
all of the ship's electrical services. On a large passenger ship with
electric propulsion, each generator may be rated at about 10 MW or more
and producing 6.6 kV, 60 Hz three-phase a.c. voltages.
The principal consumers are the two synchronous a.c. propulsion electric motors which may each demand 12 MW or more in the full away condition. Each propulsion motor has two stator windings supplied separately from the main HV switchboard via transformers and frequency converters. Thyristor convertor for voltage and thyristor invertor for frequency Modulation employed. These components are controlled by thyristor trigger control.
In an emergency a propulsion motor may therefore be operated as a half-motor with a reduced power output.
A few large induction motors are supplied at 6.6 kV from the main board with the circuit breaker acting as a direct-on-line (DOL) starting switch these are two forward thrusters and one aft thruster, and three air conditioning compressors.
Answer: (a) Sketch of diesel electric propulsion:
The principal consumers are the two synchronous a.c. propulsion electric motors which may each demand 12 MW or more in the full away condition. Each propulsion motor has two stator windings supplied separately from the main HV switchboard via transformers and frequency converters. Thyristor convertor for voltage and thyristor invertor for frequency Modulation employed. These components are controlled by thyristor trigger control.
In an emergency a propulsion motor may therefore be operated as a half-motor with a reduced power output.
A few large induction motors are supplied at 6.6 kV from the main board with the circuit breaker acting as a direct-on-line (DOL) starting switch these are two forward thrusters and one aft thruster, and three air conditioning compressors.
Harmonic filtering is used for limiting interference effects.
Other
main feeders supply the 440 V engine room sub-station (ER sub)
switchboard via step-down transformers. An inter connector cable links
the ER sub to the emergency switchboard. Other 440 V sub-stations
(accommodation, galley etc.) around the ship are supplied from the ER
sub. Some installations may feed the ships sub stations directly with HV
and step-down to 440 V locally.
The PEM drives are synchronous motors which require a controlled low voltage excitation supply current to magnetize the rotor poles. This supply is obtained from the HV switchboard via a step-down transformer but an alternative arrangement would be to obtain the excitation supply from the 440 V ER sub switchboard.
The PEM drives are synchronous motors which require a controlled low voltage excitation supply current to magnetize the rotor poles. This supply is obtained from the HV switchboard via a step-down transformer but an alternative arrangement would be to obtain the excitation supply from the 440 V ER sub switchboard.
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