A short circuit is an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally. Due to short circuit it causes the flow of excessive current in the power system leading to the interruption of power supply.
when does electric short circuit occurs?
Internal effects
Breakdown of equipment, transmission or distribution lines from deterioration of insulation in generator, transformer etc.
Inadequate design such as selecting improper equipment’s rating and improper installation.
External effects
Overloading of the equipment.
Weather conditions: It includes the insulation failure due to lightning surges. The damage of equipment due to heavy rains, heavy winds, salt deposition on overhead lines and conductors, snow and ice accumulation on transmission lines.
Smoke: If the smoke is present around the overhead lines, ionization between air and smoke particles will take place causing a flashover between the conductors or between the conductor and insulator .This flashover causes insulators to lose their insulting capacity due to high voltage.
Short Circuit in Power system
Although the power system is being designed to protect from various faults but somehow the system gets damaged. Fault current depends on the power circuit voltage and configuration, method of neutral connections (solidly grounded, resistance grounded, reactance grounded and ungrounded), presence of the regulating devices (such as shunt reactor, series reactor, shunt and series capacitors and FACT devices), and the speed of disconnection of the faulted circuit section.
Determination of fault current in power system for various fault such as 3 phase to ground,2 phase to ground, phase to phase ,single phase to ground and phase to neutral ground can be done through the short circuit analysis.
Apart from the short circuit current, the interrupting ratings of protective devices such as circuit breaker and fuses for ensuring the protection of equipment installed in the power system and as well as coordination of protective devices can be known. If an electrical fault exceeds the interrupting rate of the protective device, extensive damage of equipment will occur. Therefore any electrical equipment should not be installed without the knowledge of the complete short circuit study for the power distribution.
The short circuit in the power system cannot always be prevented but its effect can only be reduced at the time of planning and design stage of the system. The electrical equipment such as conductor, transformer, switchgear equipment (relay, circuit breaker, fuses) should be designed with the capability of withstanding the system fault current rating.
A power system is not static but changes during operation (switching on or off of generators and transmission lines) and during planning (addition of generators and transmission lines).Thus short circuit analysis should be done timely for the proper protection.
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