WHAT IS HIGH VOLTAGE:
The International Electro technical Commission describe high voltage as above (>)1000 V for alternating current(AC), & at least 1500 V for direct current(DC) & differentiate it from low voltage (50–1000 V AC or 120–1500 V DC). This is in the context of building wiring & the safety of the electrical apparatus.
In marine practice,
- voltages below (<)1,000V ac (1kV) are regarded as low voltage, &
- high voltage is any voltage above(>) 1kV. Typical marine high voltage system voltages are 3.3kV, 6.6kV & 11kV.
WHY HIGH VOLTAGE IN SHIPS:
- Higher power requirements on board ship is the foremost reason for the production of HV in ships.
- Higher power requirements have been required by the development of larger ship needed for the container transport particularly reefer containers.
- Gas carriers requires substantial cargo cooling Electrical Propulsion.
For the given power, Higher voltage(HV) means Lower current, resulting in:
- Decrease in size of generators, motors, cables etc.
- Saving of Space & weight
- Ease of Installation
- Reduction in cost of the Installation
- Lower losses – more efficient use of the generated power
- Decrease in short circuit levels in the system which determine the design & application of the electrical equipment used in the power system.
1. Higher Insulation Requirements for the cables & equipment used in the system.
2. Higher risk factor & the need for the strict adherence to stringent safety procedures
IMPORTANCE OF NEUTRAL GROUNDING :
There are many neutral grounding choice available for both Low & Medium voltage power systems. The neutral points of transformers, generators & the rotating machinery to the earth ground network allow a reference point of zero volts. This protective procedure offers many advantages over an ungrounded system, like,
- Reduced magnitude of transient over voltages
- Simplified ground fault location
- Improved system and equipment fault protection
- Reduced maintenance time and expense
- Greater safety for personnel
- Improved lightning protection
- Reduction in frequency of faults.
REASONS FOR GROUNDING OF NEUTRAL
- In the event of an Earth Fault, the fault current magnitude is sufficiently low enough such that no considerable damage is done at the fault point. This means that the faulted circuit required not to be tripped off line when the fault first occurs, it performs just like an ungrounded system.
- It can control the transient overvoltage phenomenon present on ungrounded systems if engineered properly.
- Under the earth fault conditions, the resistance should influence over the system charging capacitance but not to the point of permitting excessive current to flow and thereby excluding continuous operation.
- High Resistance Grounding(HRG) systems control the fault current when one phase of the system shorts or arcs to the ground, but at the lower levels than low resistance systems.
- In the event that a ground fault condition exists, the High Resistance Grounding typically limits the current to 5-10A.
- High Resistance Ground‘s are continuous current rated, so it does not include a time rating. Since the ground fault current is not obstructed, a ground fault detection system should be installed.
ADVANTAGES OF NEUTRAL EARTHING :
- Enables high impedance fault finding in systems with weak capacitive connection to earth
- Some phase-to-earth faults are self-cleared.
- The neutral point resistance can be selected to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage.
- It Limits phase-to-ground currents to 5-10A.
- Reduces arcing current & essentially eliminates arc-flash hazards related with the phase-to ground arcing current conditions only.
- Will eliminate the mechanical damage & may restrict thermal damage to shorted transformer & rotating machinery winding.
- Averts operation of over current devices till the fault can be located (when only one phase faults to ground).
- May be used on low voltage systems or medium voltage systems up to 5 kV. IEEE Standard 141-1993 defines that high resistance grounding could be limited to 5kV class or lower systems with charging currents of about 5.5A or less and could not be tried on 15kV systems, unless proper grounding relaying is employed.
- Conductor insulation & surge arrestors should be rated based on the line to-line voltage. Phase to neutral loads should be served through an isolation transformer.
- Generates substantial earth fault currents when combined with strong or moderate capacitive connection to earth can cause overloading for which adequate protection cost involved.
- Requires a ground fault detection system to notify that a ground fault condition has occurred.
ELECTRIC SHOCK :
Voltages greater than(>) 50 v supplied through dry unbroken human skin can cause heart fibrillation if they generate electric currents in body tissues that may pass through the chest area. Accidental contact with the high voltage providing sufficient energy may leads to severe injury or death. This can occur as a person‘s body gives the path for the current flow, causing tissue damage & heart failure. Other injuries can covers burns from the arc produced by the accidental contact. These burns can be dangerous if the victim‘s airways are affected.
HAZARDS OF HIGH VOLTAGE:
An unintentional electric arc develops during opening of the breaker, contactor or switch, when the circuit tries to keep itself in the form of an arc. During the insulation failure, when current passes through the ground or any other short circuit path in the form of accidental tool slipping between conducting surfaces, leading to short circuit.
RESULTS OF AN ELECTRIC ARC:
Temperatures at the arc terminals can reach up to 35,000° f or 20,000˚c or four times the temperature of the sun‘s surface. The heat & intense light at the point of arc is called the arc flash. Air surrounding the arc is instantly heated and the conductors are vapourised causing a pressure wave termed as ARC BLAST.
HAZARDS OF AN ARC FLASH :
- During an arc flash, instant release of large amounts of heat & light energy takes place at the point of the arc.
- Exposure frequently results in the variation of serious injuries & may even be fatal, even when the worker is ten feet or more from the arc center.
- Equipments can suffer permanent damage.
- Nearby flammable materials may be ignited leading to secondary fires.
HAZARDS OF ARC BLASTS & EJECTED MATERIALS:
- An arc flash may be followed by an arc blast
- The arc blast causes equipment to explode ejecting parts with life threatening force. Heated & vapourised conducting materials surrounding the arc expand quickly causing effects comparable to an explosive charge.
- They may project molten particles leading to eye injuries. The sound that ensues can injure the hearing.
POTENTIAL INJURIES :
- At some distance from the arc, temperatures are sometime high enough to quickly destroy skin & tissue. Skin temperatures above(>) 100˚C ( about 210˚F) for 0.1 sec result in irreversible tissue damage, also defined as an incurable burn.
- Heated air & molten materials from the arc faults can cause ordinary clothing to burn into flames even if not directly in contact with the arc Synthetic fibers may melt & adhere to the skin leading to secondary burns.
- Even when safety goggles are worn, arc flash may cause acute damage to vision & or blindness. Intense UV light produced by the arc flash can damage the retina. Pressure generated from arc blasts can also compress the eye, severely damaging vision.
- Hearing can also be affected by the loud noise & extreme pressure changes generated by the arc blasts. Sound blasts with the arc blasts exceed 140dB which is equal to an airplane taking off. Sudden pressure changes exceeding 720lbs/sq.ft for 400ms can also damage eardrums. Even at lesser pressure, serious damage to hearing may occur.