The power system is comprised of various elements such as generator, transformer, transmission lines, bus bars circuit breakers, isolators etc. Now we will discuss in brief about these elements.
1.1 Generators
The generator or alternator is the important element of power system. It is of a synchronous type and is riven by turbine thus converting mechanical energy into electrical energy. The two main parts of generator are stator and rotor. The stationary part is called stator or armature consisting of conductors embedded in the slots. The conductors carry current when load is supplied on the generator. The rotating part or rotor is mounted on the shaft and rotates inside the stator. The winding on rotor is called field winding. The field winding is excited by d.c. current. This current produces high m.m.f. The armature conductors react with the m.m.f. produced by the field winding and e.m.f. gets induced in the armature winding. The armature conductors carry current when the load is connected to an alternator. This current produces its own m.m.f. This m.m.f. interacts with the m.m.f. produced by the field winding to generate an electromagnetic torque between stator and rotor.
The d.c. current required for field winding is supplied through exciter which is nothing but a generator mounted on the same shaft on which alternator is mount. The separate d.c. source may also be used sometimes to excite the field windings through brushes bearing on slip rings.
The generators are driven by prime mover which is normally a steam or hydraulic turbine. The electromagnetic torque developed in the generator while delivering power opposes the torque provided by the prime mover.
With properly designed rotor and proper distribution of stator windings around the armature, it is possible to get pure sinusoidal voltage from the generator. This voltage is called no load generated voltage or generated voltage. The representation of generator is shown in the Fig.1.
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| Fig. 1 Representation of alternator |
1.2 Transformers
For stepping up or down the system voltage, power transformer are used in the substations. At generating end, the voltage is only stepped up for transmission of power while at all the subsequent substations the voltage is gradually stepped down to reach finally to working voltage level.
Instead of using a bank of 3 single phase transformers, a single three phase transformer is used nowadays. The advantage of using this transformer is the easiness in its installation and only one three phase load tap changing mechanism can be used.
Generally naturally cooled, oil immersed, two winding, three phase transformers upto the rating of 10 MVA are installed upon lengths of rails fixed on concrete slabs having foundations 1 to 1.5 m deep. For more than 10 MVA ratings, forced oil, water cooling and air blast cooling type may be used. The tap changers are used for regulating the voltage of transformers.
1.3 Transmission Line
1.3 Transmission Line
The Transmission line forms the connecting link between the generation stations and the distribution systems. It carries the power generated by generating stations and makes it available for distribution through distribution network.
Any electrical transmission line has four major parameters which are important from the point of view of its proper operation. These parameters are namely resistance, inductance, capacitance and conductance.
The resistance and inductance is uniformly distributed along the line. It forms series impedance. The resistance of a line is responsible for power loss. It is expected that the resistance of a line should be as low as possible so that the transmission system will be more efficient. Due to flux linkage, the conductor is associated with inductance which is distributed along the length of the line. For analysis, both resistance and inductance are assumed to be lumped.
The capacitance also exists between the conductors and is the change on the conductors per unit of potential difference between them. The conductance between conductors or between conductors and the ground due to leakage current at the insulators of overhead lines and through the insulation of cables. The leakage at conductors is negligible so the conductance between conductors of an overhead line is taken as zero. The conductance and capacitance between conductors of a single phase line or from conductor to neutral of a three phase line from the shunt admittance.
Depending upon the length of the transmission line it is classified as short transmission line, medium transmission line and long transmission line. For short line, its length is small so capacitance effects are small and are neglected.
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