We have seen that in case of interconnected systems at power stations number of alternators are operation in parallel. In fact the electricity demand of a country is fed by many alternators operating in parallel. There are various advantages of operating the alternators in parallel. Some of them are listed below.
i) For a given capacity of a generating station, either a single large unit or many small units may be installed. If there are many small units operating in parallel instead of single large unit then number of alternators operating at a time can be changed depending upon electricity requirements or load demands. This will help in operating the alternator near its full load capacity so that the efficiency will also be better. Therefore operating cost will be significantly reduced compared to a single large unit.
ii) A particular unit may be shut down for certain period during the maintenance and inspection at the power stations. For that period the load can be transferred to other units if number of units are operating in parallel.
iii) Several power stations are interconnected by a grid which is economical and advantageous. This will make sure the optimum utility of the alternators.
iv) The continuity and reliability of the supply can be maintained at better level due to interconnections.
The interconnection of alternators i.e. the process of synchronization is already discussed in previous sections. Now we will consider how the two alternators will operate in parallel. What will happen if the driving torque or the excitation or the excitation for any of the alternator is changed. In practice it is rare to have two alternators operating in parallel. But the concept of parallel operation of alternators can be will understood by considering two alternators in parallel.
Consider two identical alternators connected in parallel as shown in the Fig. 1. |
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Here cylindrical rotor alternators are assumed for simplicity, but the results obtained are equally applicable to both salient to both salient and nonsalient machines.
The terminal voltage V is given by,
From the above expression,
Solving equations (1) and (2) simultaneously,
If no load is connected to the alternators only circulating current ISY will flow in the circuit. This current is given by,
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