Saturday, May 5, 2012

Introduction to Synchronization of Alternators

       In utility systems there will be such thousands of generators which then have to be operated in parallel so that they will get interconnected by thousands of kilometers of transmission lines and will supply electrical energy to the loads which are scattered over areas of thousand kilometers. The reasons for interconnecting these systems are continuity of service, economics in plant investment and operating costs.
       When number of generators are operating at the same voltage and are required to be interconnected electrically, bus bars are used as the common electrical component. Bus bars are nothing but copper rods which operate at constant voltage.
       The process of switching of an alternator to another alternator or with a common bus bar without any interruption is called synchronization. Alternately it can also be defined as the process of connecting the two alternators in parallel without any interruption. The synchronous machine which is to be synchronized is normally called an incoming machine. If any alternators is connected to a bus bar which has many other alternators already connected, no matter what power it is supplying then alternator is said to be connected to infinite bus bar. An infinite bus bar is one of whose frequency and phase e.m.f. remains unaffected by changes in condition of any one machine connected to it. Thus they are nothing but constant frequency and constant voltage bus bars. The system can be efficiently analysed if it is connected to infinite bus bar Many important features about the behaviour of the synchronous machine can be obtained from analysis of a single machine connected to an infinite bus bar.
       In case of synchronous machines, stator carries the armature winding which is having small resistance. Under stationary conditions e.m.f. induced in stator winding is zero. So if such an alternator at stationary conditions is connected to bus bar, there is always danger of short circuit. So it is not a practice to connect a stationary to live bus bars.
1.1 Necessary Conditions for Synchronization
       To have effective synchronization without any interruption there are certain conditions to be fulfilled. These conditions are ;
i) The terminal voltage of the incoming machine must be same as that of bus bar voltage.
ii) The frequency must be same as that of the incoming machine as well as that of the bus bar. This necessitates that speed must be properly adjusted (f = PN/120).
iii) With respect to the external load, the phase of alternator voltage must be identical with that of the bus bar voltage. Alternately we can say that phase sequence for the two voltages must be same.
Note ; The violence of any of the above conditions may cause a circulating current and power surges which are accompanied by undesirable electromechanical oscillations of the rotor.
       The above conditions can be satisfied by using a voltmeter, synchronizing lamps or synchroscope. The use of voltmeter will satisfy the first conditions. Preferably the same voltmeter is used for measuring both the voltages. Bu using synchronizing lamps conditions (ii) and (iii) will be fulfilled. A synchroscope is a special device used for synchronizing the machines more accurately. It will satisfy both the conditions provided that a phase sequence indicator is used with it.
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