Load Sharing

When several alternators are required to run in parallel, it probably happens that
their rated outputs differ. In such cases it is usual to divide the total load between
them in such a way that each alternator takes the load in the same proportion of its
rated load in total rated outputs. The total load is not divided equally.
Alternatively, it may be desired to run one large alternator permanently on full
load, the fluctuations in load being borne by one or more of the others. If the
alternators are sharing the load equally the power triangles are as shown in Fig

Sharing of load when two alternators are in parallel

Consider two alternators with identical speed load characteristics connected in

parallel as shown in Fig:

Let E1, E2 be the induced emf per phase,

Z1, Z2 be the impedances per phase,
I1, I2 be the current supplied by each machine per phase

Z be the load impedance per phase,

V be the terminal voltage per phase

From the circuit we have V = E1 - I1Z1 = E2 - I2Z2 and hence, I1 = E1 - V/Z1 and
I2 = E2 - V/Z2

and also V = (I1 + I2 ) Z = IZ solving above equations

I1 = [(E1- E2) Z + E1 Z2]/ [ Z( Z1 + Z2) + Z1Z2]

I2 = [(E2- E1) Z + E2 Z1]/ [ Z( Z1 + Z2) + Z1Z2]

The total current I = I1 + I2 = [E1Z2 + E2Z1] / [ Z( Z1 + Z2) + Z1Z2]

And the circulating current or synchronizing current Is = (E1 - E2) / (Z1 + Z2)

Prime-mover Governor Characteristic

The transfer of active power between alternators in parallel is accomplished by
adjustment of the no-load speed setting of the respective prime-mover governors,
and the transfer of reactive power is accomplished by adjustment of the respective
field rheostats or voltage regulators. A typical prime-mover governor
characteristic, shown in Fig: , is a plot of prime-mover speed (or generator
frequency) vs. active power. Although usually drawn as a straight line, the actual
characteristic has a slight curve. The drooping characteristic shown in the figure
provides inherent stability of operation when paralleled with other machines.
Machines with zero droop, called isochronous machines, are inherently unstable
when operated in parallel; they are subject to unexpected load swings, unless
electrically controlled with solid-state regulators.
The no-load speed setting (and hence the no-load frequency setting) of a
synchronous generator can be changed by remote control from the generator panel
by using a remote-control switch. The switch actuates a servomotor that
repositions the no-load speed setting of the governor, raising or lowering the
characteristic without changing its slope. Curves for different no-load speed
settings are shown with broken lines in Figure
Governor Speed Regulation

Governor speed regulation (GSR) is defined as:

Effect of Change of Input Torque

The amount of power output delivered by an alternator running in parallel with
others is governed solely by the power input received from its prime mover. If two
alternators only are operating in parallel the increase in power input may be
accompanied by a minute increase in their speeds, causing a proportional rise in
frequency. This can be corrected by reducing the power input to the other
alternator, until the frequency is brought back to its original value. In practice,
when load is transferred from one alternator to another, the power input to the
alternator required to take additional load is increased, the power input to the other
alternator being simultaneously decreased. In this way, the change in power output
can be effected without measurable change in the frequency. The effect of
increasing the input to one prime mover is, thus, seen to make its alternator take an
increased share of the load, the other being relieved to a corresponding extent. The
final power-factors are also altered, since the ratio of the reactive components of
the load has also been changed. The power-factors of the two alternators can be
brought back to their original values, if desired, by adjusting the excitations of
alternators.

Effect of Change of Excitation:

A change in the excitation of an alternator running in parallel with other affects

only its KVA output; it does not affect the KW output. A change in the excitation,
thus, affects only the power factor of its output. Let two similar alternators of the
same rating be operating in parallel, receiving equal power inputs from their prime
movers. Neglecting losses, their kW outputs are therefore equal. If their excitations
are the same, they induce the same emf, and since they are in parallel their terminal
voltages are also the same. When delivering a total load of I amperes at a power-
factor of cos Ф, each alternator delivers half the total current and I1 = I2 = I/2.

Since their induced emfs are the same, there is no resultant emf acting around the
local circuit formed by their two armature windings, so that the synchronizing
current, Is, is zero. Since the armature resistance is neglected, the vector difference
between E1 = E2 and V is equal to, I1Xs1 I2Xs2 , this vector leading the current I by
900, where XS1 and XS2 are the synchronous reactances of the two alternators
respectively.

Now consider the effect of reducing the excitation of the second alternator. E2 is
therefore reduced as shown in Figure. This reduces the terminal voltage slightly, so
let the excitation of the first alternator be increased so as to bring the terminal
voltage back to its original value. Since the two alternator inputs are unchanged
and losses are neglected, the two kW outputs are the same as before. The current I2
is changed due to the change in E2, but the active components of both I1 and I2
remain unaltered. It can be observed that there is a small change in the load angles
of the two alternators, this angle being slightly increased in the case of the weakly
excited alternator and slightly decreased in the case of the strongly excited
alternator. It can also be observed that I1 + I2 = I, the total load current.
Parallel Operation of Alternators

The operation of connecting an alternator in parallel with another alternator or with

common bus-bars is known as synchronizing. Generally, alternators are used in a
power system where they are in parallel with many other alternators. It means that
the alternator is connected to a live system of constant voltage and constant
frequency. Often the electrical system, to which the alternator is connected, has
already so many alternators and loads connected to it that no matter what power is
delivered by the incoming alternator, the voltage and frequency of the system
remain the same. In that case, the alternator is said to be connected toinfinite bus-
bars.

For proper synchronization of alternators, the following four conditions must be

satisfied

1. The terminal voltage (effective) of the incoming alternator must be the

same as bus-bar voltage.

2. The speed of the incoming machine must be such that its frequency
(= PN/60) equals bus-bar frequency.

3. The phase of the alternator voltage must be identical with the phase of the
bus-bar voltage. 4. The phase angle between identical phases must be zero.

It means that the switch must be closed at (or very near) the instant the two
voltages have correct phase relationship.

Condition (1) is indicated by a voltmeter, conditions (2), (3) and (4) are indicated
by synchronizing lamps or a synchronoscope.

The synchronizing lamp method is consists of 3 lamps connected between the

phases of the running 3-ph generator and the incoming generator as shown in Fig:
In three phase alternators, it is necessary to synchronize one phase only, the other
two phases be will then synchronized automatically. However, first it is necessary
that the incoming alternator is correctly 'phased out' i.e. the phases are connected in
the proper order of R,Y &B not R, B, Y etc.

Lamp L1 is connected between R and R′, L2 between Y and B′ (not Y and Y′)
and L3 between B and Y′ (and not B and B′) as shown in Fig:

Two set of star vectors will rotate at unequal speeds if the frequencies of the two
are different. If the incoming alternator is running faster, then voltage
star R′ Y′ B′ appear to rotate anticlockwise with respect to the bus-bar voltage
star RYB at a speed corresponding to the difference between their frequencies.
With reference to Fig: 2.6, it is seen that voltage across L1 is RR′ to be increasing
from zero, and that across L2 isYB′ which is decreasing, having just passed through
its maximum, and that across L3 BY′ which is increasing and approaching its
maximum. Hence the lamps will light up one after the other in the order 2, 3,
1,2,3,1 or 1, 2, 3. If the incoming alternator is running slower, then the sequence of
light up will be 1, 3, 2. Synchronization is done at the moment the uncrossed
lamp L1 is in the middle of the dark period and other two lamps are equally bright.
Hence this method of synchronization is known as two bright one dark lamp
method.

It should be noted that synchronization by lamps is not quite accurate, because to a

large extent, it depends on the sense of correct judgment of the operator. Hence, to
eliminate the element of personal judgment in routine operation of alternators, the
machines are synchronized by a more accurate device called a synchronoscope as
shown in Fig: It consists of 3 stationary coils and a rotating iron vane which is
attached to a pointer. Out of three coils, a pair is connected to one phase of the line
and the other to the corresponding machine terminals, potential transformer being
usually used. The pointer moves to one side or the other from its vertical position
depending on whether the incoming machine is too fast or too slow. For correct
speed, the pointer points vertically up.
SYNCHRONOUS GENERATOR

1. What do you mean by the salient-pole type rotor?

Salient - pole type rotor means a low and moderate speed rotor having large
diameter and small axial length with projected poles coming out of the rotor frame
the outer surface of which almost follows the inner cylindrical surface of the stator
frame.

2. Define voltage regulation of an alternator

The voltage regulation of an alternator is defined as the increase in terminal

voltage when full load is thrown off, assuming field current and speed remaining
the same.

Percentage regulation = (E0 – V) /V x 100

E0 = No load terminal voltage
V = Full load rated terminal voltage.

3. What are the advantages of having rotating field system?

1. Better insulation
2. Ease of current collection
3. Increased armature tooth strength.
4. More rigid construction
5. Reduced armature leakage reactance.
6. Lesser number of sliprings.
7. Lesser rotor weight &intertia
8. Improved ventilation & heat dissipation.
4. Why is EMF method called Pessimistic method?

The value of voltage regulation obtained by EMF method is always more than the
actual value, therefore it is called Pessimistic method.

5. Why is MMF method called Optimistic method?

Compared to the EMF method, MMF method, involves more number of complex
calculation steps. Further the OCC is referred twice and SCC is referred once while
predetermining the voltage regulation for each load condition. Reference of OCC
takes care of saturation effect. As this method require more effort, the final result is
very close to the actual value. Hence this method is called optimistic method.

6. Compare salient pole rotor & smooth cylindrical rotor.

Salient Pole Rotor

1 Large diameter and short axial
2. Used for low speed alternators
3.Has projecting poles
4. Needs damper windings
5.Windage loss is more

Cylindrical Rotor
1 . Small diameter and long axial length, length
2. Used for high - speed turboalternators
3. No projecting poles
4. Does not need damper windings.
5. Windage loss is less

7.How is the armature winding in alternators different from those used in dc

machines?
The armature winding of the alternator is placed in the stator, but the in case of dc
machines, armature winding is placed in rotor.

8. What are the methods by using zero p.f. lagging curve can be obtained?
Zero power factor characteristic of an alternator gives the variation of terminal
voltage with field current, when the alternator is delivering its full rated current to
a zero power factor (lagging) load. This characteristic is obtained by running the
machine at synchronous speed and connecting a purely inductive

3phase load to its terminals. The load is varied in steps and at each step the field
current is adjusted, so that the armature current is equal to its rated value.

9. What are squirrel-cage windings of alternators? How and why are they
used?

Damper windings are squirrel cage windings of the alternators.

This winding is placed in rotor pole shoes

10. Write down the equation for frequency of emf induced in an Altenator.

Frequency of emf induced in an Alternator,f ,expressed in cycles per second or Hz,

is given by the following equation

F = (PN)/120 Hz,

Where
P- Number of poles
N-Speed in rpm
11. What are the advantages of salient pole type construction used
forSynchronous machines?
Advantages of salient-pole type construction are : They allow better ventilation
The pole faces are so shaped that the radial air gap length increases from the pole
center to the pole tips so that the flux distribution in the air-gap is sinusoidal in
shape which will help the machine to generate sinusoidal emf
Due to the variable reluctance the machine develops additional reluctance power
which is independent of excitation

12. Name the types of Alternator based on their rotor construction.

Alternators can be classified into the following two types according to its rotor
construction
Smooth cylindrical type alternator
Salient pole alternator
13.Why is short pitch winding preferred over full-pitch winding ?

Advantages
Waveform of the emf can be approximately made to a sine wave and distorting
harmonics can be reduced or totally eliminated.

Conductor material , copper , is saved in the back and front end connections due to
less coil-span.

Fractional slot winding with fractional number of lots/phase can be used which in
turn reduces the tooth ripples.

Mechanical strength of the coil is increased.

14. Define winding factor.
The winding factor Kd is defined as the ratio of phasor addition of emf induced in
all the coils belonging to each phase winding to their arithmetic addition.

15. Why are Alternators rated in kVA and not in kW?

The continuous power rating of any machine is generally defined as the power the
machine or apparatus can deliver for a continuous period so that the losses incurred
in the machine gives rise to a steady temperature rise not exceeding the limit
prescribed by the insulation class

Apart from the constant loss incurred in Alternators is the copper loss, occurring in
the 3 -phase winding which depends on I2 R, the square of the current delivered by
the generator.

As the current is directly related to apparent - power delivered by the generator ,

the Alternators have only their apparent power in VA/kVA/MVA as their power
rating.

16.What is the necessity for predetermination of voltage regulation?

Most of the Alternators are manufactured with large power rating ,hundreds of kW
or MW, and also with large voltage rating upto 33kV. For Alternators of such
power and voltage ratings conducting load test is not possible. Hence other indirect
methods of testing are used and the performance like voltage regulation then can
be predetermined at any desired load currents and power factors.

17. Name the various methods for predetermining the voltage regulation of 3-
phase Alternator.

The following are the three methods which are used to predetermine the voltage
regulation of smooth cylindrical type Alternators
Synchronous impedance / EMF method Ampere-turn / MMF method
Potier / ZPF method

18. What are the advantages and disadvantages of estimating the voltage
regulation of an Alternator by EMF method?

Simple no load tests (for obtaining OCC and SCC) are to be conducted
Calculation procedure is much simpler
Disadvantages:
The value of voltage regulation obtained by this method is always higher than the
actual value

19. What are the test data required for predetermining the voltage regulation
of an Alternator by MMF method?
Data required for MMF method are :

Effective resistance per phase of the 3-phase winding R Open circuit

characteristic (OCC) at rated speed/frequency Short circuit characteristic (SCC) at
rated speed/frequency

22. State the condition to be satisfied before connecting two alternators in

parallel.

The following are the three conditions to be satisfied by synchronizing the

additional Alternator with the existing one or the common bus-bars.

The terminal voltage magnitude of the incoming Alternator must be made equal to
the existing Alternator or the bus-bar voltage magnitude.
The phase sequence of the incoming Alternator voltage must be similar to the bus-
bar voltage. The frequency of the incoming Alternator voltage must be the same as
the bus-bar voltage.

23.List the factors that affect the load sharing in parallel operating
generators?

The total active and reactive power delivered to the load, connected across the
common bus-bars, are shared among Synchronous generators, operating in parallel,
based on the following three factors
Prime-mover characteristic/input

Excitation level and

Percentage synchronous impedance and its R/X ratio

24.Why almost all large size Synchronous machines are constructed with
rotating field system type?

The following are the principal advantages of the rotating field system type
construction of Synchronous machines:

The relatively small amount of power, about 2%, required for field system via slip-
rings and brushes. For the same air gap dimensions, which is normally decided by
the kVA rating, more space is available in the stator part of the machine for
providing more insulation to the system of conductors, especially for machines
rated for 11kV or above. Insulation to stationary system of conductors is not
subjected to mechanical stresses due to centrifugal action.

Stationary system of conductors can easily be braced to prevent deformation. It is

easy to provide cooling arrangement for a stationary system of conductors.
Firm stationary connection between external circuit and system of conductors
enable he machine to handle large amount of volt-ampere as high as 500MVA.

25.Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high speeds.
The hig h speed operation of rotors tends to increase mechanical losses and so the
rotors should have a smooth external surface. Hence, smooth cylindrical type
rotors with less diamete rand large axial length are used for Synchronous
generators driven by steam turbines with either 2 or4 poles.

26.Which type of Synchronous generators are used in Hydro-electric plants

and why?

As the speed of operation is low for hydro turbines used in Hydro-electric plants,
salient pole type Synchronous generators are used. These allow better ventilatio
nandalso have other advantages over smooth cylindrical type rotor.

27.How does electrical degree differ from mechanical degree?

Mechanical degree is the unit for accounting the angle between two points based
on their mechanical or physical placement. Electrical degree is used to account the
angle between two points in rotating electrical machines. Since all electrical
machines operate with the help of magnetic
fields,theelectricaldegreeisaccountedwithreferencetothemagneticfield.180 electrical
degree is accounted as the angle between adjacent North and South poles.

28.What is distributed winding?

When coil-sides belonging to each phase are housedor distributed in more than
ones lot under each pole regi on then the winding is called distributed winding. A
full pitch coil has width of coil other wise called coil-span as 180º-angle between
adjacent slots in electrical degree and x=1,2,3…

29.Definewinding factor.

ThewindingfactorKdisdefinedastheratioofphasoradditionofemfinducedinallthe
coilsbelonging toeach phasewinding to theirarithmeticaddition.

30.What are the causes of changes in voltage in Alternators when loaded?

VariationsinterminalvoltageinAlternatorsonloadconditionareduetothefollowing
three causes:
□ Voltage variation due to the resistance of the winding, R
□ Voltage variation due to the leakage reactance of the
winding, Xt

31.What is meant by armature reaction in Alternators?

Theinteractionbetweenfluxsetupbythecurrentcarryingarmatureandthemainis defined
asthe armature reaction.

32.What do you mean by synchronous reactance?

Synchronous reactance Xs=(Xl+Xa) The value of leakage reactance Xl

Is constant for a machine based on its construction. Xa depends on saturating

condition of the machine. It is the addition of Xa, which represent the armature
reaction effect between two synchronously acting magnetic fields that makes the
total reactance Xa to be called synhronous reactance.

33.What ismeant by synchronous impedance of an Alternator?

The complex addition of resistance, Rand synchronous reactance, jXs can be

represented together by a single complex impedance Zs called synchronous
impedance.

In complex form Zs = (R + jXs )In polar form Zs = |Zs| <Where |Zs|= ¥_52 + X2 s)

34.What is meant by load angle of an Alternator?

The phase angle introduced between the induced emf phasor, E and terminal
voltage phasor , U during the load condition of an Alternator is called load angle.
35.Why is the stator core of Alternator laminated?

The stator core of Alternator is laminated to reduce eddy current loss.

36.State the condition to be satisfied before connecting two alternators in

parallel

The following are the three conditions to be satisfied by synchronizing the

additional Alternator with the existing one or the common bus-bars.

□ The terminal voltage magnitude of the incoming Alternator

must be made equal to the existing Alternator or the bus-bar voltage magnitude.

□ The phase sequence of the incoming Alternator voltage

must be similar to the bus-bar voltage.

□ The frequency of the in coming Alternator voltage must be

the same as the bus-bar voltage.

37.How do the synchronizing lamps indicate the correctness of phase sequence

between existing and incoming Alternators?

The correctness of the phase sequence can be checked by looking at the three sets
of lamps connected across the3-pole of the synchronizing witch. If the lamps grow
bright and darkinunisonitisan indication of the correctness of the phase sequence.
If on the other hand, they become bright and dark one after the other, connections
to any two machine terminals have to be interchanged after shutting down the
machine.
38.Whataretheadvantagesanddisadvantagesofthreedarklampsmethodofsynchr
onizing?

Advantages:
□ The synchronous switch using lamps is inexpensive

□ Checking for correctness of the phase sequence can be

obtained in a
simplemannerwhichisessentialespeciallywhentheAlternatorisconnectedforthefirst
time or for fresh operation after disconnection .

Disadvantages:

□ The rate of flickering of the lamps only indicates the frequency difference
between the bus-barand the incoming Alternator. The frequency of the in coming
Alternator in relation to the bus-bar frequency is not available.

39.Howsynchronoscope is used for synchronizing Alternators?

Synchronoscope can be used for permanently connected Alternators where the

correctness of phase sequence is already checked by other means. Synchronoscope
is capable of rotating in both directions. The rate of rotation of the pointer
indicates the amount of frequency difference between the Alternators. The
direction of rotation indicates whether incoming Alternator frequency is higher or
lower than the existing Alternator. The TPSTs witch is closed to synchronize the
incoming Alternator when the pointer faces the top thick line marking.
40.Why synchronous generators are to be constructed with more synchronous
reactance and negligible resistance?

The presence of more resistance in the Synchronous generators will resist or

oppose their synchronous operation. More reactance in the generators can cause
good reaction between the two and help the generatorstoremainin
synchronisminspite of any disturbance occurring in any one of the generators.

41.List the factors that affect the load sharing in parallel operating
generators?

The total active and reactive power delivered to the load, connected across the
common bus-bars, are shared among Synchronous generators, operating in parallel,
based on the following three factors

□ Prime-mover characteristic/input
□ Excitation level and
□ Percentage synchronous impedance and its R/X ratio

42. How does the change in prime mover input affect the load sharing?

An increase in prime-mover input to a particular generator causes the active

power shared by it to increase and a corresponding decrease inactive- power shared

by other generators. The change in reactive power sharing is less appreciable. The
frequency of the bus-bar voltage will also subjected to slight increase in value.
43. How does change in excitation affects the load sharing?

The decrease in excitation in one generator causes the reactive power shared byitto
decrease and a corresponding increase in reactive-power shared by other
generators. The change inactive-power sharing is less appreciable. There will be a
slight decrease in terminal voltage magnitude also.

44.What steps are to be taken before disconnecting one Alternator from

parallel operation?

The following steps are to be taken before disconnecting one Alternator from
parallel operation

The prime-mover input of the outgoing generator has to be decreased and that of
other generators has to be increased and by this the entire active-power delivered
by the outgoing generator is transferred to other generators.

□ The excitation of the outgoing generator has to be

decreased and that of other generators have to be increased and by this the entire re
active-power delivered by the outgoing generator is transferred to other generators.

□ After ensuring the current delivered by the outgoing

generator is zero,it has to be disconnected from parallel operation.

45.What is meant by infinite bus-bars?

The source or supply lines with non-variable voltage and frequency are called
infinite bus-bars. The source lines are said to have zero source impedance and
infinite rotational inertia.
46.How does increase in excitation of the Alternator connected to infinite bus-
bars affect this operation?

Increase in excitation level of the synchronous generator will effectively increase

the reactive component of the current supplied by the generator and hence the
active power delivered.

47.Upon what factors does the load angle depend?

Angle is positive during generator operation and negative during motor operation.

48.An Alternator is found to have its terminal voltage on load condition more
than that on no load. What is the nature of the load connected?

The nature of the load is of leading power factor ,load consisting of resistance and
capacitive reactance.