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. |
| إضافة تسمية توضيحية |
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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