In the previous post we have
seen that synchronous generators do not operate individually in a
generating station but they are interconnected so that total generating
capacity will be high.When number of alternators are interconnected
forming a system which may be treated as an infinite bus. Infinite bus
bar is one which keeps constant voltage and frequency although the load
varies. Thus it may behave like a voltage source with zero internal
impedance and infinite rotational inertia. Any alternator switched on to
or off, the infinite bus does not cause any change in the voltage and
frequency of the system.

The
characteristics of a synchronous generator on infinite bus bars are
quite different from those when it is connected to another alternator
and both are in parallel. When two alternators are connected in parallel
we have seen that a change in the excitation changes the terminal
voltage and p.f. is determined by load. However change in excitation for
an alternator connected to infinite bus bar will not change the
terminal voltage but the power factor only is affected whereas the power
developed by an alternator depends only on mechanical power input.

Now
we will consider the effect of excitation and driving torque on the
performance of an alternator which is connected to infinite bus bar. In
all the further discussion we will take zero losses for the machine.

__1.1 Effect of Excitation__

where E = Induced e.m.f. or excitation e.m.f.

V = Constant bus voltage

I = Armature current

Z

_{s }= Synchronous impedance
Again we will consider the two cases one with alternator on no load and other with alternator on load.

__1.1.1 Alternator on No Load__

Since
we are considering the losses to be zero the power angle will be zero.
Thus the power transferred from or to the bus is zero ( P = ((EV/X

_{s}) sinδ )
Now
if the excitation is properly adjusted at no load then induced e.m.f. E
will be equal to bus voltage V and no current will flow. This is shown
in Fig. 2. This is floating condition of alterntor.

Fig. 2 Fig.3 |

Now
if the alternator is under excited then induced e.m.f. E will be less
that V. This will cause circulating current I

_{SY}to flow which will lead E by angle of 90^{o}. Due to this it produces magnetizing m.m.f. which will try to increase field m.m.f. to maintain alternator terminal voltage equal to the bus bar voltage. This is shown in the following Fig. 3.
Similarly
if alternator is over excited then induced e.m.f. will be more than V
which will again cause a circulating current to I

_{SY}flow. The power angle δ is zero. This current lags E by 90^{o}. This will produce demagnetizing armature m.m.f. which will counterbalance the effect of increased field m.m.f. and again the terminal voltage of an alternator will be equal to constant bus bar voltage V. This is represented in Fig. 4.Fig. 4 |

It
can be seen that in both the cases considered above, no active power is
delivered since I

_{SY}is in quadrature with V and load angle is also zero. But alternator takes reactive power from bus since E < V and delivers it to bus if E > V.__1.1.2 Alternator on Load__

Now
let us consider that alternator is supplying power to an infinite bus
which has induced e.m.f. E, power angle δ and working at unity power
factor with current I.

With
mechanical power input to the alternator remaining constant, the power
given by (EV/X

_{s}) sin δ will remain constant. If by varying excitation induced e.m.f. E is increased to E_{1}then the load angle will also change from δ to δ_{1}. From the phasor diagram it can be determined as E_{1}sinδ_{1}= E sinδ as V and X_{s}are constant. The drop due to synchronous reactance also increases and armature current increases from I to I_{1}. This current has two components one real component and other quadrature component. This quadrature component is nothing but demagnetizing component. This will result in lagging power factor cosΦ_{1}.
Similarly
if the excitation is decreased so that induced e.m.f. reduces from E to E

_{2}with corresponding change in power angle from δ to δ_{2}. The armature current in this case will be I_{2}which has real component and magnetizing component which results in leading power factor cosΦ_{2}. This can be represented in the phasor diagram shown in Fig. 5.Fig. 5 |

From the phasor diagram it can be seen that

I

_{1}cosΦ_{1}= I_{2}cosΦ_{2}= 1
Multiplying by V throughout,

V I

_{1 }cosΦ_{1}= V I_{2}cosΦ_{2}= VI
This
indicates that power delivered to the bus will remain constant. Thus by
changing the field excitation the active power is unaltered. But change
in excitation results in corresponding operating power factor as shown
in phasor diagram.

**Note**: An under excited alternator operates at leading power factor whereas an over excited alternator operates at lagging power factor.

It
can also be seen that armature current is minimum at unity power
factor. For over excited alternator as E

_{1}cos δ >V therefore as seen from case (i) i.e. no load condition alternator delivers reactive power to the bus whereas for underexcited alternator E_{2}cosδ < V, alternators takes reactive power from the bus. This variation of excitation and armature current can be plotted as shown in Fig. 6. This is known as curves for synchronous generators by virture of their shape.Fig. 6 V curves for alternators |

__1.2 Effect of Driving Torque__

As
already discussed in the previous section the driving torque of an
alternator can be changed by throttle opening in steam power plants and
by gate opening in case of hydrogeneration. Let us see the effect of
driving torque on performance of alternator with the help of phasor
diagram as shown in Fig. 7.

Fig. 7 |

The voltage equation remains same as

The
load angle is δ. Now if the driving torque of alternator is increased
keeping excitation constant then output (EV/X

_{s}) sinδ also increases as is changing, but E, V and X_{s }are constant. The angle increases so as to balance between increased mechanical input and the power (EV/X_{s})sinδ. Thus the tip of phase E follows a curved path. The maximum value of δ will be 90^{o}for which arm,armature current is I_{1}leading the bus bar voltage V by power factor angle Φ_{1}.
Thus
with increase in input the alternator delivers more power to infinite
bus. The frequency and terminal voltage of an alternator remains same as
it is connected to infinite bus bar.

If
driving torque is decreased, the power angle δ must decrease
correspondingly. If it becomes zero, no power is transferred to the
infinite bus. The prime mover will only supply the losses.

If driving torque is reversed or if the prime mover is decoupled from the shaft E shifts and lags behind V, then δ will be reversed and the operation of machine will change from synchronous generator to synchronous motor as now

Th synchronous motor operates at a leading p.f. indicating that it is delivering reactive power to infinite bus.

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ReplyDeleteI want synchronization generator not alternator

ReplyDeleteBest ans and,BT sir explain abt generator.

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