Question

It is proposed to operate a Bow thruster unit from a 3.3 KV electrical supply.

Outline suitable options for the design of installation under each of the following heading. (16)

(a) Protection of main switch board

(b) Overload of a bow thruster motor

(c) Cable protection

Answer 1

Part (a)

Design a separate HV side panel for the bow thrusters, isolated from adjacent LV panels.
Ensure independent earthing system for the HV side, connecting to the ship's hull.
Incorporate harmonic filters on both ends of the HV busbar to avoid waveform distortion.
MSB should be robust to withstand maritime conditions such as vibration, humidity, high temperatures, and exposure to sea water.
Implement a neutral earth resistor (NER) for neutral point earthing.
Choose circuit breakers suitable for HV, such as vacuum or SF6 types, with manual or motor-driven closing mechanisms.
Integrate standard interlocks and trips for under voltage, overcurrent, and short circuit protection.
Include monitoring instruments for incoming voltage, frequency, current, in/out power lamps, and load status lamps.
Additional features like anti-condensation heaters and interlocks for main-emergency power and door closure are recommended.

Part (b)

Utilize a combined motor protection relay offering overcurrent (inverse time relay), differential current monitoring, and earth fault monitoring.
Connect the relay to a 'Lock-out' relay to trip the motor power in the event of overload.
Implement thermistor trip/protection for single phasing and high-temperature conditions.

Part (c)

Ensure proper cable rating based on the electrical system specifications (1900/3300V for earthed neutral system; 3300/3300V for insulated neutral system).
Choose conductors rated for short circuit and maximum fault current.
Select high-quality insulation materials such as EPR (ethylene propylene rubber) or XLPE (cross-linked polyethylene).
Use sheet material with ratings for short circuit, heat, oil, chemical resistance, and flame retardancy.
Plan cable tray routing and ensure proper junctions with terminals.
Consider the complexity of insulation design for HV cables, emphasizing clearance and creepage distance.
Recognize that less copper area is required for HV conductors, allowing for space and weight savings during installation.

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