Marine High Voltage

High Voltage in Ships

We all know about the voltages used on board a ship. It is usually a 3phase, 60Hz, 440 Volts supply being generated and distributed on board. Every day the owners and designers aim for bigger ships for more profitability. As the ship size increases, there is a need to install more powerful engines and other machineries. This increase in size of machineries and other equipment demands more electrical power and thus it is required to use higher voltages on board a ship. Any Voltage used on board a ship if less than 1kV (1000 V) then it is called as LV (Low Voltage) system and any voltage above 1kV is termed as High Voltage. Typical Marine HV systems operate usually at 3.3kV or 6.6kV. Passenger Liners like QE2 operate at 10kV.

DEFINITION FOR HIGH VOLTAGE 

The numerical definition of high voltage depends on context. Two factors considered in classifying a voltage as “high voltage” are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer to the voltage between two conductors of a system, or between any conductor and ground. In electric power transmission engineering, HIGH VOLTAGE is usually considered any voltage over approximately 33,000 volts. This classification is based on the design of apparatus and insulation.

The International Electro technical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current and distinguish it from low voltage (50–1000 V AC or 120–1500 V DC) and extra-low voltage (<50 V AC or <120 V DC) circuits. This is in the context of building wiring and the safety of electrical apparatus.

In the United States 2005 National Electrical Code (NEC), high voltage is any voltage over 600 V (article 490.2).

 British Standard BS 7671:2008 defines high voltage as any voltage difference between conductors that is higher than 1000 V AC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 V AC or 900 V ripple-free DC.

What is classed as high voltage?
In marine practice.Voltages below 1,000Vac (1kV) are considered low voltage, and

•  High voltage is any voltage above 1kV. Typical marine high voltage system voltages are 3.3kV, 6.6kV and 11kV.

The major differences between high voltage supply and low voltage supply on board ships are:

• High voltage systems are more extensive with complex networks and connections.
• Isolated equipment must be earthed down.
• Access to high voltage areas should be strictly limited and controlled.
• Isolation procedures are more involved.
• Switching strategies should be formulated and recorded.
• Specific high voltage test probes and instruments must be used.
• Diagnostic insulation resistance testing is necessary.
• High voltage systems are usually earthed neutral and use current limiting resistors.
• Special high voltage circuit breakers have to be installed.

WHY HIGH VOLTAGE IS USED ON BOARD SHIPS

• Higher power requirements on board vessels are the foremost reason for the evolution of HV in ships.
• Higher power requirements have been necessitated by development of larger vessels required for container transport particularly reefer containers.
• Gas carriers needing extensive cargo cooling Electrical Propulsion.
• For ships with a large electrical power demand it is necessary to utilize the benefits of a high voltage (HV) installation.
• The design benefits relate to the simple ohms law relationship that current (for a given power) is reduced as the voltage is increased. Working at high voltage significantly reduces the relative overall size and weight of electrical power equipment.

As per ohms law:
POWER = VOLTAGE x CURRENT
For a given Power, Higher the Voltage, Lesser is the Current
440 KW = 440,000 Watts
                            = 440 Volts  x 1000 Amps
                            =1100 Volts  x 400 Amps
                            =11000 Volts x 40 Amps

When large loads are connected to the LV system the magnitude of current flow becomes too large resulting in overheating due to high iron and copper losses.
  P = VI CosФ
                               Copper loss =I² R [kW]
HV levels of 3.3 kV, 6.6 kV and 11 kV are regularly employed ashore for regional power distribution and industrial motor drives.

For example, a motor (let us assume a bow thruster), may be of a smaller size if it designed to operate on 6600 Volts.

For the same power, the motor would be of a smaller size if it is designed for 6600Volts when compared to 440Volts.

Thus these are the major reasons why recent ships have shifted towards high voltage systems.

The main disadvantage perceived by the user /maintainer, when working in an HV installation, is the very necessary adherence to stringent safety procedures.
 

Advantages/disadvantages of using HV advantages

Advantages
For a given power, higher voltage means Lower current, resulting in:

•  Reduction in size of generators, motors, cables etc.
•  Saving of Space and weight.
•  Ease of Installation.
•  Reduction in cost of Installation.
• Lower losses, more efficient utilization of generated power.
• Reduction in short circuit levels in the system which decides the design and application of the electrical equipment used in the power system.

Disadvantages

• Higher Insulation Requirements for cables and equipment used in the system.

Higher risk factor and the necessity for strict adherence to