Introduction:
Electrical energy is generated in large scale in generating station for the sake of economy.There are different type of generating station but must of the large power station are hydro-electric type and also low cost .Usually the generating station are situated far away from the local center (cities)theretofore extensive power supply networks between generating stations together with power supply networks and loads is known as power system.Basically a power system consist of following four major stations.
1.Generating station
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| Power System |
2.Transmission system
3.Distribution system and
4.Loads
1)Generating station:
A generating station may have one or more number of generators connected in parallel.The generators are driven by the turbines.The turbine may be various type such as -water turbine ,gas turbine ,steam turbine ,petrol turbine etc.Usually generator generate power at 11kv,3-phase system .The power generated at the generating stations are transmitted to the consumers at the load centers through the transmission and distribution lines.
2)Transmission line:
Transmission line transmits bulk amount of power from the generating station to the sub-station or one sub-station to the another sub-station without applying any consumers in routes. The power is transmitted at the high voltage level in order to reduce line current ,which in turn reduces size of conductor in transmission line and power loss in the line .The transmission line can be classified in to two types namely-Primary transmission line and sub-transmission line .The transmission line just after the generating station ,is known as primary transmission line whereas the transmission line connecting the two sub-stations ,is known as sub-transmission line .
A country may have many number of grid substation and they are usually inter- connected through the tri line (transmission line)to form a national grid.
3)Distribution system:
At the receiving end of the sub-transmission line ,the voltage level is further stepped down to the 11kv (may be other values in different country ) and distribution system starts from here .The distribution system is divided into the two sections -namely primary distribution system and secondary distribution system . The 11kv line is known as primary distribution system .It is also known as feeder.The distribution line supplies power to big consumer such as hotel, factory commercial complex etc and they will have their own distribution transformer and secondary distribution system .The primary distribution line also supplies power to 11kv/400v distribution transformers at different location.from the secondary side of these transformer the secondary distribution line (3-phase ,4wire)starts.The small consumer are supplied by the single phase line from these 3-phase ,4 wire system .
4)Loads:
The various power consuming equipment at consumer premises such as light ,heater ,electric fan etc are known as loads.These loads may have different types .The following two types are the most common in power system ,one is constant impedance load and other is constant power load.
The power rectifier thyristors is known as silicon controller rectifiers(SRC) which are commonly used is adjustable rectifier circuits , mostly in high power units up to 100 MVA in power electronics .If the positive voltages is supplied without gate current , the thyristors constitutes the state of forward bias blocking .A low power signal or pulse of the gate current play to switch the thyristors to the on states . The output characteristic of the thyristors in the forward bias is similar to the curve that of the normal diode with small leakage current in forward bias configuration . The thyristors show very low resistance in the forward bias connection .Once it is turn ON the thyristors remain in this state after the end of gate signal or pulse while its current is is higher than the holding level state.If the current drops below the holding level current state the component or devices turn back on the OFF state or it is known as non conducting state.