1.0 Introduction
An electric power system is made up of generation, transmission, distribution and consumer equipment (loads). The system must be protected against flow of heavy short-circuit currents which can cause permanent damage to the major equipments. And this is achieved by disconnecting the faulty section of the system by means of circuit breakers, switch gears, isolators and protective relays.
The need to know the maximum three phase short-circuit current that can occur at the different points of the system in order that the breakers elected are adequately to withstand the current and operate successfully, to cut the faulty section, and also in order that the protective relays may be selected for correct operation. The design of machines, bus bars, isolators, circuit breakers etc is based on the consideration of normal and short-circuits currents.
It is also important to be able to calculate approximately at least the size of the protective reactors which must be inserted in the system to limit the short circuit current to a value which can be handled by the circuit breakers.
The three phase (3-Ø) fault currents is an A.C system are determined mainly by the reactance of the alternators, transformers, and lines up to fault in the case of phase-to-phase faults. When the fault is between phase and earth, the resistance of the path plays an important role in limiting the current.
The rupturing capacities of a circuit breakers are based on the symmetrical short-circuit current which is the most simple calculation among all types of short-circuits. However, for determination of setting of relays, it is absolutely necessary to know fault current due to unsymmetrical fault condition for which knowledge of symmetrical components etc is required.
1.1 Transmission Lines
Electric power transmission is a process in the delivery of electricity to consumers is this bulk transfer of electrical power form the source point to the various consumers. A power transmission network typically connects power plants to multiple substations near a populated area.
Transmission lines are the media which the transmission process is achieved. The lines used, composed of wires of copper-clad or aluminum-clad steel, which are suspended form tall lattice work towers of steel by strings of porcelain insulators or glass insulators by the use of clad steel wires and high tower, the distance between towers can be increased, the cost of transmission lines thus reduced.
Electricity transmission lines carry power of the voltages of 11KV and above to reduce the energy loss on transmission. They are transmitted as alternating current through overhead power transmission lines.
1.2.0 Modelling of Transmission Line
The transmission line is the main energy corridor in a power system. It is modeled based on the following consideration.
1.2.1 Short Transmission Line
A short transmission line is line less than 80km, due to the distance of line, the effect of capacitance is not considered. Its performance depends on resistance and inductance of the line, the equivalent circuit diagram is shown below.
The material contains electrical diagrams, circuits, illustrations and more (Very detailed)
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