It has already been seen that in the case of interconnected systems at the power stations the parallel operation of two alternators.In fact, the electricity demand of a country is fed by many alternators operating in parallel. There are various advantages of Parallel Operation Of Two Alternators or Synchronous Generators.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 the single large unit then a number of alternators operating at a time can be changed depending upon electricity requirements or load demands.This will in operating the alternator near its hill load capacity so that efficiency will also be better.Therefore operating cost will be significantly reduced compared to the 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 the number of units is 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.
There are certain requirements which must be met for parallel operation of alternators be reliable.
1) The rated speeds of the alternators should be such that they give same frequency generated the voltage (f =PNs/120)
2) The output voltage rating of the alternators must be same.
3) Even though the kVA ratings of the alternators are different, they should be of the same type so that waveform of generated voltage is same.
4) The armature of alternators should have reactance.
5) The prime movers of the alternators should have same speed load drooping characteristics so that they can be loaded in proportion to their output ratings.
This conditions for proper synchronization of the alternators for the parallel operation to be successful are as given below.The alternator or group of alternators which are already in operation and load sharing are called running machines while the alternator which is to be connected to these alternators for parallel operation is called incoming machine.The incoming machine must meet the following conditions for proper parallel operation of alternators.
1) The terminal voltage of the incoming machine must be exactly same as that of the others or of the bus bars connecting them.
2) The speed of the incoming machine must be such that its frequency if- PNs 120 equals the frequency of other machines or that of the bus bar.
3) The phase of the incoming machine voltage must be same as that of the other machines or bus bar with respect to load.The phase voltages of incoming machine and bus bar should be in opposition which ensures that there is no circulating current between windings of alternators already in operation and the incoming machine.
The interconnection of alternators i.e. the process of synchronization is already discussed previously.Now we will consider how the two alternators will operate parallel.What will happen if the driving torque 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 well understood by considering two alternators in parallel.
Parallel Operation Of Alternators or Synchronous Generators – Diagram:
Consider two identical alternators in parallel operation as shown in below figure.
Here cylindrical rotor alternators are assumed for simplicity, but the results obtained are equally applicable to both salient and non-salient machines.
Solving two equations,
If no load is connected to the alternators only circulating current Isy will flow in the circuit.The current is given by