Marine high voltage systems are classified based on voltage levels:
- AC Systems: High voltage is classified as any voltage above 1000 volts (1 kV).
- DC Systems: High voltage classification starts at 1500 volts (1.5 kV).
- Typical marine high-voltage systems operate at standard levels such as 3.3 kV, 6.6 kV, and 11 kV.
Features of Marine HV System:
- Neutral is always earthed through a Neutral Earthing Resistor (NER) to limit earth fault current.
- HV systems are more expensive than LV systems due to the need for special insulation, protective gear, and safety features.
- They carry a higher arc-flash hazard, necessitating stringent operational and maintenance safety procedures.
- The general layout of an HV system is similar to an LV system, but includes additional protective and safety components.
Ships High Voltage Distribution System:
- 6.6 kV Generator Sets: These generate the high voltage power.
- High Voltage Switchboards: These contain switchgear, protection devices, and instrumentation for controlling and monitoring the HV system.
- HV Cables: These carry high-voltage power throughout the ship.
- High to Low Voltage Step-Down Transformers: These reduce the high voltage to lower voltages suitable for various loads.
- High to High Voltage Step-Down Transformers (e.g., 6.6 kV to 2.9 kV): These may be used to step down voltage between different parts of the system.
- High Voltage Motors: These are used for propulsion and other high-power applications.
- Harmonic Filters: These mitigate harmonic distortion in the system.
- Earthed Neutral (NER): This provides a safety ground for the system.
Protective Devices in Marine HV Systems:
Protective Device | Function |
Overcurrent (Instantaneous) | Trips the breaker immediately on high current to protect equipment. |
OCIT (Overcurrent Inverse Time) | Shortens the trip delay as overcurrent magnitude increases. |
Earth Leakage | Detects small earth faults and trips the system to prevent damage or fire. |
Reverse Power Protection | Prevents motorization of generators by detecting reverse current flow. |
Undervoltage Protection | Trips equipment if the supply voltage drops below safe limits. |
Overtemperature Protection | Used to monitor cable and equipment temperatures to prevent overheating. |
Differential Fault Protection | Compares phase currents (inlet and outlet); trips if imbalance detected. |
Thermal Overload (Thermal O/L) | Trips on excessive current over time to protect insulation and windings. |
Locked Rotor Protection | Detects if a motor rotor is stalled by checking imbalance across phases. |