The power rating of ship electrical equipment applies to all equipment (electrical or mechanical) commonly found in electrical power plants.
The electrical power ratings of generators and transformers are stated in kVA or MVA (not in the real power kW or MW).
Calculation of real power on ship generator
These machines can deliver rated voltage and rated current, but the real power delivered by the machine depends on the power factor, which is primarily determined by the nature of the load connected.
We often see the ship electrical power plant capacity expressed in MW, since it refers to the mechanical output rating of the prime mover, which is the dominant investment in the power plant. Moreover, the MW rating also determines the ship plant’s fuel consumption rate, such as the number of coal cars coming daily in the thermal power plant. That is why the MW rating draws more attention than the MVA rating.
- Nameplate rating refers to the full load output, and input = output + internal losses.
- Efficiency = output power/input power, which varies with the output power.
- Cable rating is stated in terms of the voltage and the current-carrying capacity (ampacity).
- The mechanical power rating of motor or engine is stated in hp in British units or kW in SI units.
- Equipment rating is limited by the operating temperature limit on insulation in electrical equipment and the cooling method used in the design.
Service factor and overload on ship equipment
The service factor (SF), indicated on the equipment nameplate, is the equipment’s ability to carry temporary overload without thermal or mechanical failure or measurable reduction in life. Industry standards require all electrical equipment to have an SF of at least 1.15 typically for 2 hours.
It means the equipment can carry 15% overload for 2 h. However, the SF is often misinterpreted as the factor by which we can continuously overload the motor by 15%. If it is continuously overloaded by 15%, the equipment may run hotter than the design temperature by more than 10°C, reducing the service life to less than one-half, which is a significant penalty to pay.
