In the name of ALLAH, the

most Beneficent, the most

Merciful

MUHAMMAD SHAHID SHAUAKAT Project Engineer

ORIENT ENERGY SYSTEMS PVT LTD LAHORE

Introduction

Alternator Construction

Excitation

Speed of rotation of a synchronous generator

Internal Generated Voltages

Equivalent Circuit of Synchronous Generator

Phasor Diagrams of Synchronous Generator in case of

load

Power in Synchronous Generator

Synchronous Generator operating alone:

Effect of load change on a synchronous generator:

With unity power factor

With lagging power factor

With leading power factor

Voltage Regulation

Parallel Operation of Alternator with Large Power

System

Parallel Operation of Alternator with another Generator

Alternators are synchronous machines used to convert

mechanical energy to ac electric power.

Alternators generate electricity using the principle, when the

magnetic field around a conductor changes, a current is

induced in the conductor.

Typically, a rotating magnet, called the rotor turns within a

stationary set of conductors wound in coils on an iron core,

called the stator. The field cuts across the conductors,

generating an induced EMF (electromotive force)

The rotor's magnetic field may be produced by induction (as

in a "brushless" alternator), by permanent magnets (as in very

small machines), or by a rotor winding energized with direct

current through slip rings and brushes.

In a synchronous generator, a dc current is applied to the rotor winding, which produces a rotor magnetic field.

The rotor of the alternator is then turned by a prime mover, producing a rotating magnetic field within the machine.

This rotating magnetic field then induces a three phase set of voltages within the stator windings of the generator.

An electric generator or electric motor that uses field coils

rather than permanent magnets requires a current to be

present in the field coils for the device to be able to work.

If the field coils are not powered, the rotor in a generator can

spin without producing any usable electrical energy, while the

rotor of a motor may not spin at all.

The process of generating a magnetic field by means of an

electric current is called excitation.

Since the rotor is rotating, there are common two approaches to

supply dc current to rotor winding.

1. Supply dc power from an external source to the rotor by

means of slip rings and brushes.

2. Supply dc power from a special dc power source mounted

directly on the shaft of alternator (brushless supply).

A brushless exciter is a small ac generator with its field circuit

mounted on the stator and its armature circuit mounted on the

rotor shaft. Then the three phase output of this generator is

rectified to supply the dc power to rotor field.

A brushless exciter circuit

Brushless exciter including a pilot exciter

In synchronous generators the electrical frequency is locked in or synchronizes with the mechanical rate of rotation of the generator.

The rate of rotation of the magnetic field in the machine is related to the stator electrical frequency as,

𝒇𝒆 =𝒏𝒎𝑷

𝟏𝟐𝟎

Where

› 𝑓𝑒= electrical frequency, in Hz

› 𝑛𝑚= mechanical speed of magnetic field in r/min

› 𝑃= number of poles

The magnitude of the internally generated stator

voltage is as follows.

𝑬𝑨 = √𝟐𝝅𝑵𝒄𝝓𝒇,

› NC= number of stator coils

This voltage depends on the flux in the machine,

the frequency or speed of rotor, and the

machine’s construction.

EA is the internally generated voltage in on phase of the

synchronous generated.

However this voltage is not usually the voltage that appears at

the terminals Vϕ of the generator.

There are number of factors that cause the difference between

EA and Vϕ.

1. The distortion of the air-gap magnetic field by the current

flowing in the stator, called “armature reaction”.

2. The self-inductance of the armature coils.

3. The resistance of the armature coils.

So, the phase voltage will be expressed as,

𝑽∅ = 𝑬𝑨 + 𝑬𝒔𝒕𝒂𝒕𝒐𝒓

Here 𝑬𝑺𝒕𝒂𝒕𝒐𝒓 is the voltage induced due to the armature

reaction, which is,

𝑬𝑺𝒕𝒂𝒕𝒐𝒓 = −𝒋𝑿𝑰𝑨

So,

𝑽∅ = 𝑬𝑨 − 𝒋𝑿𝑰𝑨

In addition to the armature reaction, the stator coils have a

self-inductance LA (and reactance XA) and the resistance RA,

then the terminal voltages are,

𝑽∅ = 𝑬𝑨 − 𝒋𝑿𝑰𝑨 − 𝒋𝑿𝑨𝑰𝑨 − 𝑹𝑨𝑰𝑨

If

XS= XA+ X= synchronous reactance

Then,

𝑽∅ = 𝑬𝑨 − 𝑹𝑨𝑰𝑨 − 𝒋𝑿𝑺𝑰𝑨

Phasor Diagram of alternator is the graphical

representation of the voltage drops occur in the

generator.

It shows the relation of Internally generated

voltage EA, terminal voltage Vϕ and the voltage

drops.

Phasor diagram of an alternator at unity power factor

IA

EA

IARA

jXSIA

VΦ

Phasor diagram of alternator at lagging power factor

IA

EA

IARA

jXSIA

VΦ

Phasor diagram of alternator at leading power factor

IA

EA

IARA

jXSIA

VΦ

For a given phase voltage and armature current, a larger

internally generated voltage EA is needed for lagging loads

than for leading loads.

Therefore a larger field current is needed with lagging loads

to get same terminal voltages, as

EA=Kϕω

And ω must be constant to keep a constant frequency.

In other words, for a given field current and magnitude of

load current, the terminal voltage is lower for lagging loads

and higher for leading loads.

In real synchronous machines, the synchronous reactance is

much larger than the winding resistance RA, so RA is often

neglected in the qualitative study of voltage variations.

Pconverted

Pout

=√ τindωm

Pin=τ

appω

m

I2R losses

Copper losses Core losses

Frictional

& windage

losses

Stray

losse

s

a b

O θ

γ

δ

EAsinδ

=XSIAcosθ θ

Vϕ

The input mechanical power is the shaft power in the generator

Pin=τappωm.

The power converted from mechanical power to electrical power is

given by,

Pconverted=τindωm =3EAIAcosγ

Where “γ” is the angle between EA and IA. The real electrical power

of the synchronous generator is

Pout = √𝟑𝐕𝐓𝐈𝐋 𝐜𝐨𝐬 𝛉=3VϕIAcosθ

and the reactive power is

Qout =√𝟑𝐕𝐓𝐈𝐋 𝐬𝐢𝐧 𝛉.

In the figure shown above the vertical segment “bc” can be

expressed as EAsinδ or XSIAcosθ. So,

𝑰𝑨 𝐜𝐨𝐬 𝜽 =𝑬𝑨𝒔𝒊𝒏𝜹

𝑿𝑺

So, the output power will be

𝑷𝒐𝒖𝒕 =𝟑𝑽∅𝑬𝑨𝒔𝒊𝒏𝜹

𝑿𝑺

The behavior of synchronous generator under load varies

greatly depending on the power factor of the load and on

whether the generator is operating alone or in parallel with

other synchronous generators.

Generator Load

An increase in the load is an increase in the real and/or reactive power drawn from the generator.

Such a load increase increases the load current drawn from the generator. Because the field resistor has not been changed, the field current is same as earlier, and therefore the flux ϕ is constant.

Since the prime mover also keeps a constant speed ω, the magnitude of the internal generated voltage EA=Kϕω is constant.

The effect of changing the load of generator in this case the behavior of generator will be changed for different power factors.

In the case of lagging power factor when more load is added

at the same power factor, then |IA| increases but remaining at

the same angle as earlier.

Therefore, the armature reaction voltage jXSIA is larger than

before but at the same angle. As,

𝑬𝑨 = 𝑽∅ + 𝒋𝑿𝑺𝑰𝑨

or

𝑽∅ = 𝑬𝑨 − 𝒋𝑿𝑰𝑨

So, Vϕ will decrease in the case of load adding with lagging

power factor.

jXSI’

A

I’A VΦ

E’A

IA

EA

jXS I

A

V’Φ

V’Φ

E’A

IA

EA

VΦ

I’A

A conventional way to compare the voltage behavior

of two generators is by their voltage regulation. The

voltage regulation of a generator is defined by the

equation,

𝑽𝑹 =𝑽𝒏𝒍 − 𝑽𝒇𝒍

𝑽𝒇𝒍 𝒙 𝟏𝟎𝟎%

Where Vnl is the no load voltage and Vfl is the full load

voltage of the generator.

A generator operating at a lagging power factor has a

large positive voltage regulation.

A generator operating at a unity power factor has a

small positive voltage regulation.

A generator operating at leading power factor often

has a negative voltage regulation.

It is desired to keep the voltage supplied to a load

constant, even though the load varies. The obvious

approach is to vary the magnitude of EA to

compromise for the change in load.

As EA=Kϕω, since the frequency should not be

changed in a normal system; EA must be controlled

by varying the flux in the machine.

The idea to regulate the terminal voltages can be

summarized as follows:

By changing the field resistance RF, field current can

be changed IF.

Change in field current IF will change the flux ϕ in

the machine.

Change in flux ϕ will change the internal generated

voltages EA.

This process will change the output voltage Vϕ as,

𝑽∅ = 𝑬𝑨 − 𝑹𝑨𝑰𝑨 − 𝒋𝑿𝑺𝑰𝑨

More than one generator operating in parallel to

supply the power demand to the load is called

parallel operation of alternators.

Several generators can supply a bigger load

than single generator.

Having many generators increases the

reliability of the power system.

Having many generators operating in parallel

allows one or more generators to be removed

in case of failure of preventive maintenance.

RMS line voltages of the generators must be

equal.

Generators must have the same phase sequence.

Phase angles of the two “a” phases must be

equal.

The frequency of the oncoming generator must

be slightly higher than the frequency of running

system.

As the power drawn from prime movers increases, the speed at which they turn decreases.

Whatever the governor mechanism is present, it will always be adjusted to provide a slight drooping characteristics with increasing load,

𝑺𝒑𝒆𝒆𝒅 𝑫𝒓𝒐𝒑 = 𝑺𝑫 =𝒏𝒏𝒍 − 𝒏𝒇𝒍

𝒏𝒇𝒍𝒙 𝟏𝟎𝟎%

nnl= no-load prime mover speed

nfl= full-load prime mover speed

Most generators prime movers have a speed drop of 2 to 4 percent.

nnl

Me

ch

an

ica

l Sp

ee

d r

/min

nfl

Pfl Power KW E

lec

tric

al Fre

qu

en

cy

Hz

ffl

Pfl Power KW

fnl

P = sp(fnl - fsys) › sp = slop of curve in kW/Hz

› fsys=operating frequency of system

fe= nmP/120

› fe=electrical frequency

› nm=mechanical speed

VTnl

Te

rmin

al V

olta

ge

s V

T

VTfl

Qfl Reactive Power kVAR

Q = sp(VTnl - VTsys) › sp = slop of curve in kVAR/V

› VTsys=operating voltage of system

For any given real power, the governor set points

control the generator’s operating frequency fc.

For any given reactive power, the field current

controls the generator’s terminal voltages VT

When an alternator is connected to a power system,

the power system is often so large that nothing the

operator of the generator does will have much of an

effect on the power system.

A large power system is the system

› Terminal voltages are constant

› Electrical frequency is constant

Ter

min

al

Vo

lta

ges

VT

VT

Reactive Power kVAR Q

Ele

ctri

cal

freq

uen

cy

fe

Active Power kW P

PG PInfinite bus

fnl

P kW

fe

Pload

If Oncoming generator’s frequency

is greater than the frequency of

running system

PG < 0 +P kW

fe

-P kW

If Oncoming generator’s frequency is

less than the frequency of running

system

For the increase in the real power sharing the

governor’s set point of the generator is increased.

For the increase in the reactive power sharing the

field current of the generator is increased.

PG1 PB1

fnl

PG kW

fe

Pload= PG+PB

PG2 PG3 PB2 PB3 PB kW

When a generator is operating in parallel with a

larger system (infinite bus):

The frequency and terminal voltages of generator are

controlled by the system to which it is connected.

The governor set point of the generator control the

real power supplied by the generator.

The field current in the generator controls the

reactive power supplied by the generator

If a generator is connected in parallel with

another generator then the basic constraint is that

the sum of real and reactive power supplied by

the generators must be equal the demand by load.

Pload= PG1+PG2+PG3

Qload= QG1+QG2+QG3

System frequency and power supplied by a single

generator is not constant.

When two generators are operating in parallel, an

increase in governor set points on anyone of them,

Increases the system frequency.

Increases the power supplied by that generator ,

while reducing the power supplied by the other

generator.

PG2 PG1

f1

kW

fe

Pload= PG1+PG2

kW

f2

Effect of change of Governor’s set point

When two generators are operating in parallel, an

increase in field current of anyone of them,

Increases the system terminal voltages.

Increases the reactive power supplied by that

generator , while reducing the reactive power

supplied by the other generator.

QG2 QG1

V1

kVAR2

VT

Qload= QG1+QG2

kVAR1

V2

Effect of change of Field Current

An increase in governor’s set point increases the system

frequency and the power supplied of that machine.

To adjust the power sharing without changing the system

frequency, increase the governor set points of one generator

and simultaneously decrease the governor set points of other

generator.

PG2 PG1 kW

fe

Pload= PG1+PG2

kW

fsys

An increase in field current increases the system voltage and

the reactive power supplied of that machine.

To adjust the power sharing without changing the system

voltages, increase the field current of one generator and

simultaneously decrease the field current of other generator.

QG2 QG1 kVAR 2

VT

Qload= QG1+QG2

kVAR

1

Vsys

In the case of two generators operating in parallel,

The system is constrained in that the total power supplied by

the two generators must be equal to the load demand.

System frequency fsys and terminal voltages VT are not

constant in this system.

To adjust the real power sharing between generators without

changing system frequency, simultaneously increase the

governor set point of one generator and decrease governor set

points of other generator.

To adjust the system frequency without changing the power

sharing simultaneously increase or decrease the governor set

points of both generators.

To adjust the reactive power sharing without changing the

system voltage simultaneously increase the field current of

one generator and decrease the field current of other

generator.

To adjust the system voltage without changing the reactive

power sharing simultaneously increase or decrease the field

current of both generators.