HIGH VOLTAGE SYSTEMS ON SHIPS
- As we all know usually the ship operates with a 3 phase. 60Hz. 440 Volts supply which is 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 machinery. This increase in size or machinery and other equipment demands more electrical power and thus it is required to use higher voltages on board a ship.
What is High Voltage?
- In Marine Practice majority of merchant ships have a 3-phase 3 wire, 440 V insulated neutral earth power systems. This power system falls in the category Of Low Voltage and meets the power demands of medium capacity motors up to 200 kW.
- Voltages up to & Including 1000V is known as Low voltage system and Voltages 1001 and above is called as High Voltage system in maritime practice for ships. However voltages up to 11kV are called as medium voltage in land based practices.
Definition of High Voltage
- The numerical definition of high voltage depends on context, Two factors considered 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 any 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?
- In marine practice, Voltages below 1000 VAC (1kV) are considered low voltage, and high voltage is any voltage above 1 kV. Typical marine high voltage System voltages are 3.3 kV, 6.6 kV and 11kV.
THE MAJOR DIFFERENCES BETWEEN HIGH VOLTAGE SUPPLY AND LOW VOLTAGE SUPPLY ON BOARD SHIPS ARE:
1. High voltage systems are more extensive with complex networks and connections,
2. Isolated equipment MUST BE earthed down
3. Access to high voltage areas should be strictly limited and controlled
4. Isolation procedures are more involved
5. Switching strategies should be formulated and recorded
6. Specific high voltage test probes and instruments must be used
7. Diagnostic insulation resistance testing is necessary
8. High voltage systems are usually earthed neutral and use current limiting
9. Special high voltage circuit breakers have to be installed
Why High Voltage in Ships?
– Higher power requirements onboard vessels is the foremost reason for the evolution of HV in ships.
– Higher power requirements have been necessitated by development of larger vessels required for container transort particularly for reefer containers.
– gas carriers needing extensive 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 x40 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= V I cosΦ
Copper Loss = I2 R
- MV levels of 3.3 k V, 6.6 k V and 11 k V are regularly employed ashore for regional power distribution and industrial motor
- 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 6600 Volts when compared to 440 Volts, 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
— 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 utilisation 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.
1. Higher Insulation requirements for cables and equipment used in the system.
2. Higher risk factor and the necessity for strict adherence to stringent safety procedures.