Synchronous Generator Fundamentals and Operation

Generators Electrical Maintenance

Synchronous Generator: Fundamentals & Operation Training Module

ALSTOM (Switzerland) Ltd ABCD

Generators Synchronous Generator: Electrical Maintenance Fundamentals & Operation ABCD

We reserve all rights in this document and in the information contained therein. Reproduc-tion, use or disclosure to third parties without express authority is strictly forbidden. Copyright by ALSTOM (Switzerland) Ltd 2004

This is a training module. All mentioned / attached values and settings are for information only. The values might be changed during commissioning. For actual values please refer to the Instruction Manual.

Power Plant Training Center CSXA220052enB.doc Page 2

Generators Synchronous Generator: Electrical Maintenance Fundamentals & Operation ABC

Static Excitation ..................................................................................................................................................15

D

Table of Contents

Module Objectives...................................................................................................................................................4

Introduction ..............................................................................................................................................................5 Generator Types...................................................................................................................................................5

Generator Fundamentals........................................................................................................................................6 Conductor in a Magnetic Field .............................................................................................................................6 Synchronous Generator .......................................................................................................................................6 Reminder ............................................................................................................................................................10

Operation ................................................................................................................................................................11 Operating Range - Power Chart ........................................................................................................................11 Other Generator Charts .....................................................................................................................................13

Generator Excitation .............................................................................................................................................14 Brushless Excitation ...........................................................................................................................................14

Automatic Voltage Regulator (AVR)....................................................................................................................15

Summary.................................................................................................................................................................16

Figure 1: Salient and Non-salient Pole Generator.............................................................................................17

Figure 2: Synchronous Generators.....................................................................................................................18

Figure 3: Generation of a Single Phase Alternating Voltage...........................................................................19

Figure 4: Principal Arrangement of Stator Windings .......................................................................................20

Figure 5: Magnetic Field in a Generator at No-load Condition........................................................................21

Figure 6: Magnetic Field in a Generator at Load Condition.............................................................................22

Figure 7: Synchronous Generator: Equivalent Circuit Diagram, Over-excited and Under-excited Vector Diagram.............................................................................23

Figure 8: Vector Diagram and Power Chart .......................................................................................................24

Figure 9: Open Circuit and Short Circuit Curves ..............................................................................................25

Figure 10: Power Chart of a Turbogenerator: Operating Ranges and Limits ...............................................26

Figure 11: Brushless and Static Excitation Systems .......................................................................................27

Figure 12: Major Components of a Typical Brushless Excitation System ....................................................28



Module Objectives

Upon completion of this section, the trainee is able to:

Explain how voltage is induced: in a conductor rotating in a magnetic field in a synchronous generator.

Draw the flow of the resulting magnetic field: at no-load operation at load operation.

Identify 3 conditions that must be met for a generator to produce electrical voltage.

Using the power chart and generator data sheet: Find normal operating point and mark it in the power chart. Find for any operating points the associated approximate field & stator currents.

Explain the mechanical analogy of the load angle.

By help of power chart & the above mentioned nature of the load angle list and explain the limiting characteristics.

Describe with a drawing and with few words the excitation system as regards: its purpose list and explain the working principle of the two different types of excitation explain the two regulator types.

Explain the AVR's behaviour when the actual generator output voltage changes; in case the excitation is: in the auto channel or in the manual channel.

List different types of synchronous generator and describe their main features.



Introduction

The synchronous generator is the most important machine used for producing electrical en-

ergy for the following reasons: The output voltage or the reactive power output can be controlled by a variable DC cur-

rent applied to the machine from a control unit, the Automatic Voltage Regulator (AVR). In parallel operation with the network the reactive- and active power output can be set

independently from one another over a wide range.

Generator Types

Figures 1 & 2 Salient & Non-Salient Pole Generators

There are two basic forms of Generators which differ mainly in their rotor design: The first, called a salient-pole machine, has protruding field poles, each pole provided

with a concentrated field winding. This type is used mostly on hydro-electric plant.

The second type of machine (non-salient pole), generally called turbogenerator has a smooth cylindrical rotor. The field winding is placed in slots distributed over a specific zone on the circumference of the rotor. This type of generator is used mostly on thermal power plants.

The turbo generator was invented in 1901 by Charles Brown of the Brown Boveri Co.

In the following we mainly refer to the 2-Pole Turbogenerator



Generator Fundamentals Conductor in a Magnetic Field

Figure 3 If a coil is rotating in a magnetic field as shown in the Figure 3 an (alternating, induced) volt-

age can be measured at the sliprings. A voltage is induced because the flux (linked with the coil) is varying with time.

dtdWU =

timetFluxmagnetic

turnswindingW

Flux is proportional to the flux density B (flux per unit area) and the surface within the coil loop.

dAB = windingcoilwithinareasurfaceA densityfluxB If the coil is turning with uniform speed (in this homogenous field) a sinusoidal voltage is

induced.

Synchronous Generator

In praxis the construction is different than discussed above:

The armature winding (the winding in which voltage is induced) is placed in the stator. The magnetic field is produced by the rotor which is rotating

The advantage of this approach is: Voltage, current on the rotating part are of lower magnitude. This will effect the design of

the generator.

Figure 4.1 The magnetic field is produced by a DC current flowing through the winding in the rotor.

Through the arrangement of field winding the field over the poles has a sinusoidal shape which is slightly stepped (valid for the turbogenerator)

The field is moving (because of its rotation) relative to the stationary stator winding Therefore the flux through the winding is changing and a voltage is induced in the arma-

ture/ stator winding as a consequence.

Figures 4.1 & 4.2 By special arrangement of the stator windings around the circumference with

multiple winding turns per phase and displacement of each of the three winding set by 120 degrees

a practically sinusoidal 3-phase voltage (phase displaced in time by 120 degrees) is produced. The Figure 4.2 shows a simplified winding arrangement with three winding loops per phase.

(Continued next page)



Synchronous Generator (continued)

Multiple Pole Generator - Operation Speed

Above we mainly referred to a two pole generator type: During one revolution every wind-ing on the stator sees the north pole & the south pole once. With a speed of 3000 rpm we get 50 revolutions in one second which corresponds to 50Hz of the induced voltage. If we have more poles, e.g. four, the stator windings see two north and two south poles during one revolution. The speed would be reduced to 1500 rpm in this case for the same grid frequency.

Figure 5 Magnetic Field in a Generator at no load

Figure 5 shows the magnetic field at no load operation of the generator (no current flowing in the stator windings). We see the closed magnetic circuit, through rotor, air gap and back through stator core (shown by closed lines (circles); the total number of line would corre-spond to the flux).

Figure 6.1 Under load condition

As soon as the generator is on grid (the load connected to generator terminals): the currents flowing in every phase of the stator windings generate their one magnetic

fields. The sum of the three is called the armature reaction field, rotor and armature reaction field form the total field. It can be imagined as armature or

stator field.

Figure 6.2 Load Angle (simplified view)

The angle between the rotor pole axis and the total field axis ("stator field") is the Load Angle . To illustrate this, we can imagine two vectors (arrows) rotating around a centre point O: one stands for the

rotor field the other for the stator field (total resulting field). They are strongly coupled with each other. The rotor is actually pulling the stator field

(the grid) behind. The stator field is rotating therefore with the same speed as the rotor, called synchronous

speed. What gave the name to this type of machine: Synchronous Generator.





Figure 7.1 Simplified Equivalent Circuit Diagram

In order to better understand the electrical operating behaviour of a generator, an "Equivalent Circuit Diagram" is used. In the diagram (where only one phase is shown), the generator is represented by the (internal) Pole Wheel Voltage UP and its synchronous reactance Xd.

U1 (= UN / 3) is the phase voltage of the generator and I1 the current flowing in one of the stator windings. (The ohmic resistance of the stator winding is very small in relation to Xd ( 0.5 .. 2 per mil) and is therefore neglected.)

At no load

the stator current I1 is 0. The voltage Ud over Xd is therefore 0, the Terminal Voltage the same as the Pole Wheel Voltage UP. (UP = U1).

If a load is connected to the terminals U1 and Up is not the same any longer. There will be a phase displacement between U1 and I1.

Figure 7.2 For clarification we look at the phasor or vector diagram of two special cases (which do not

appear during normal operation): Vector Diagram for a mere Inductive Load First we have a pure inductive load (no ohmic resistance). The current I1 is lagging the voltage U1 by 90 degrees (as seen from the grid).

Figure 7.3 For a mere Capacitive Load In the second case we have a pure capacitive load connected to the generator terminals. The current is I1 is leading the voltage U1 by 90 degrees (as seen from the grid).

Figure 8.1 Vector Diagram for a normal Operating Mode (Ohmic-Inductive Load)

Under normal operating condition the generator feeds a load composed of two parts: a bigger ohmic part (which is e.g. converted into heat at the consumer side) and a smaller inductive part (which is e.g. used for the magnetisation of motors). The corresponding vector diagram is shown in the figure. The load angle with the above mentioned explanation - angle between rotor and stator field, pulling the stator field behind - can still be seen. Of importance is the next step:





Figure 8.2 Turning into Power Chart

We divide the length of the vectors (of the diagram) by the synchronous reactance Xd and rotate the vector diagram by -90 degrees: As a result without going into the details - we obtain, out of the voltage vector diagram, a current vector diagram. The current vectors are adapted in such a way that they fit into the power chart of the generator.

The Power Chart

The Power Chart represents the actual operating range of a generator. A point within the power chart stands for a momentary operating point. Point N in the figure represents one special point (out of many possible points): The nor-

mal operating point for which the generator is designed. The power chart is scaled for 1 per unit (p.u.) and is valid for 1 p.u. voltage referring to

the nominal data of the generator. (The plant specific nominal data of the generator may be seized down compared to the original data of the generator to meet e.g. the power data of the main step up transformer).

Figure 8.2 The normal Operating Point within the Power Chart

The normal operating point N in the Power Chart has a distance from the centre 0 which represents the nominal Apparent Power (in 1 p.u.). Projected to the axis we can read out: the nominal Reactive Power part on the x-axis and the nominal Active Power part on the y-axis.

As describe above we can overlay the vector diagram of the currents:

Horizontally to the left of the centre 0 we find the No-load Field Current; it is used to magnetise the generator (it corresponds to the field current which is needed

to maintain nominal voltage at no-load operation of the generator). Starting point is 1/ Xd. (valid for the a Turbogenerator only).

The Nominal Field Current is found as vector from point 1/ Xd to N and the Nominal Stator Current from point 0 to N. Also the Load Angle and the Phase Angle (displacement between phase voltage U1

and stator current I1) is to be found in the diagram.





Point N can be derived from the data sheet of the generator. Also the values for the nomi-

nal currents and the synchronous reactance can be found there. Be aware that you have to scale the field currents and the stator current separately. This

because the current vector diagram is composed by the vectors: stator current I1 and the other two If0' and If' which are proportional to the actual values (If0 and If).

To become familiar with the findings refer to the various exercises included in the training program.

Figure 9 Open Circuit Characteristic

The Open Circuit Curve is measured during no-load operation. The terminal voltage is recorded against different field current values. The voltage at the generator output is nearly proportional to the field current, being bent

at higher voltage because the iron core becomes saturated.

Short Circuit Characteristic The Short Circuit Curve is taken while the terminals are shorted (Terminal voltage U1 = 0).

The stator current is recorded against different field current values. The curve is a straight line.

The two curves are highly depending on the design of the generator (and they characterise the generator). Also the synchronous reactance can be derived from these curves.

Reminder

Conditions needed for Production of Electrical Voltage

A rotating generator rotor will produce electrical voltage if three things are present: Magnetic field. Conductor in the stator. Relative motion between the two.



Operation Operating Range - Power Chart

Figure 8.2 Data of any Operating Point

The power and current values for any operating point can be derived from the power chart the same way as discussed above. The operating points are normally given by their active and reactive power values or the

apparent power and power factor values (given one pair value the other can be directly read out of the power chart).

Furthermore, from the operating point the corresponding field and stator currents can be deduced. This simply by putting the vector lengths of the new point in relation with the reference vectors of point N.

Refer to the exercise during training for clarification.

Accuracy of the Readings

The readings are approximate values only. The drawing resolution plays a role. Another factor is the following: By changing the field current it is possible to regulate the reactive power output, the terminal voltage may change as well (depending on the grid strength and plant design). As we already know the power chart is only valid for Nominal Voltage (= 1 p.u.). Remark: The reactive power reading would change with the square of the terminal voltage.

Figure 10.2 Overexcited Power Range

As an example we look at a generator on the grid. We follow an operating point T in the Power Chart along an horizontal line. That means the power output at the turbine stays the same. The manipulations for the experiment are done at the Excitation System only (from local control panel or from remote)! We start in the right quarter of the power chart at point T1.

The machine is overexcited, phase angle inductive, lagging, power factor positive (> 0); the generator produces reactive power.

Only Active Power Output If we decrease the field current we come to a point T2

where the reactive power output is zero, the phase angle = 0 (voltage and current are in phase),

the load angle has become bigger from T1 to T2

Underexcited Power Range If the field current is further decreased we come into the underexcited region to a point

T3 (left quarter), the phase angle becomes leading, capacitive; the power factor negative (


Operating Range - Power Chart (continued)

Figure 10.1 Underexcited Limiting Range

At the point where the load angle becomes 90 degrees (with low field current), we reach an unstable operating point. Remember the spring/ rubber band between rotor and stator field. At 90 degrees the torque is undefined and the machine starts to slip ("goes out of step",

"loses" synchronism). The regulation of the field current actually defines the coupling strength between the ro-

tor and stator field. To prevent this status the P/Q Limiter (or Load Angle Limiter) limits the load angle by

limiting the reactive power Q depending on the momentary load

(according a preprogrammed curve; P/Q-Limiter, for newer Excitation Systems) or the load angle

(one straight inclined line; Load Angle Limiter, for older Excitation Systems) in both cases the field current is kept above a certain value by the Excitation System.

Maximum Field / Rotor Current Limitation

The field current is limited by the maximum allowed heat production in the windings (de-pending on the ohmic resistance, the amount of current flowing in the conductor and the cooling efficiency of the cooling system). The field current is normally limited at 105% of rated value (for continuous operation). In the Power Chart this is actually marked with an arc from C to E (of 100% in length and

centre point 1/ Xd (Turbogenerator)).

Stator Current Limitation

Is the maximum allowed stator current (depending on ohmic resistance, current flow and cooling efficiency) normally set at 105% of its rated value; limiting arc C-D with centre A.

Change of Current Limitations

Both Rotor and Stator Limitations may be temperature dependent (depending on the Cold Gas Temperature (temperature of coolant at inlet)).

More over the two limiters normally have a dynamic respond characteristic. With short time (overrated) higher currents the Generator / Excitation System can respond to de-mands/ faults outside of its system.




Operating Range - Power Chart (continued)

Limitation and Protection

The above limiters are built in functions (in newer system implemented within the soft-ware) of the excitation system: they are normally part of the automatic channel only (voltage controlled channel)

Because the manual channel is "field current controlled" (regulation according a preset field current) and has normally no (or only few rudimentary) limiters special care have to be taken by the personnel to operate the generator within its operating range by changing field current set point (during load changes a.s.o., see also chapter Automatic Voltage Regulator)

Limiters are backed up by protection functions (beyond the limiters' range). They are im-plemented in the excitation system (e.g. excitation overcurrent), generator protection system (e.g. loss of excitation, underfrequency) or higher control system

(with alarm or trip: stator winding temperature or hot gas temperature too high). Triggered protection functions will initiate one or several of following measures:

deload the generator (until cause of disturbance is cleared), take generator off grid, deexcitate the generator, shut down the turbine.

Other Generator Charts

V-Curve

The V-Curve shows the correlation between field and stator current for a certain active load (normally for 25, 50, 75, 100% load; every load level represents

one curve). At the same time it indicates the power factor levels along the curve.

Unbalanced Load Curve

The generator is designed for a symmetrical 3-phase load (three symmetrical currents). Asymmetrical current components will produce an inverse rotating field. This induces

currents in damper winding, resulting in losses and possible overheating. It is therefore not desired.

The unbalanced load is defined as the ratio of counter current component to the rated cur-rent, in percent. The Unbalanced Load Curve expresses this ratio over time during which this operation is allowed at most. For continuous operation the maximum allowed ratio lies between 6 to 8%.




Other Generator Charts (continued)

Other Charts

There are various (with the generators data less frequently enclosed) charts. For example: Generator efficiency (efficiency as a function of active power output & power factor) Generator losses (as a function of active power output & power factor) Heat dissipation (as a function of active power output & power factor) Generator capability (active power output) versus cooling water or air inlet temperature Chart with operating range of voltage and frequency

Generator Excitation

Purpose of Excitation

As we have seen above, we need a field current regulation to produce and regulate the mag-netic field in the rotor. There are two ways to bring this current to the rotor windings: by brushless excitation system or by static excitation system. The two are shortly described below.

Brushless Excitation

Figures 11 & 12 Design and Operating Principle of Brushless Excitation System

When a brushless excitation system is used, the actual field current is produced by an exciter machine: an armature machine coupled to the generator shaft

(Armature: is the part of the machine where voltage is induced). The exciter machine has a 3-phase winding in the rotor and a DC field winding in the

stator. The induced voltage, current in the rotor is rectified by rotating diodes (also on the ro-

tor) and the resulting DC current is fed to the field winding of the main generator. The excitation system (AVR) feeds the stationary field winding of the exciter machine.

The regulator is either supplied from the generator terminals (via excitation transformer) or from a pilot exciter (permanent generator) located at the end of the generator shaft.

The terminal voltage is therefore indirectly regulated. The control circuit has a relative big time constant

(compared to the static excitation system; see on the following page)



Static Excitation

Figure 11 Working Principle

In this case the field current is supplied to the rotor winding by the static excitation system via brushes an slip-ring devices.

The excitation is usually shunt supplied, that means it is fed via the excitation trans-former from its own generator terminals.

The rectifier is composed of one or more thyristor bridges (in parallel). Therefore its name: Static, no moving parts.

The static excitation system has a faster dynamic response characteristic than the brushless excitation system. The ceiling voltage required (max. needed voltage) for the generator rotor is determined

by the ratio of the excitation transformer which has an influence on the response time of the excitation system.

Automatic Voltage Regulator (AVR)

Figure 11 The term is a bit misleading because most of the time it is used for the whole controller sys-

tem (either voltage or field current controlled).

Automatic Voltage Regulator (AVR)

With the actual meaning: the automatic voltage controller; in most cases referred to as Auto Channel. It regulates the generator terminal voltage to a given set point value.

If external factors changes (as e.g. load or grid conditions), the AVR automatically ad-justs the field current to reach the preset voltage value.

The AVR may be superimposed by a power factor controller (maintaining a given power factor) or reactive power controller (maintaining a given power factor).

The superimposed regulator acts on the AVR-set point and is slower than the AVR




Automatic Voltage Regulator (AVR) (continued)

Field Current Regulator (FCR)

The field current regulator or referred to as manual channel. It regulates the field current to a constant value given by a set point regardless of the gen-

erator terminal voltage. If the load increases for example the reactive power output drops simultaneously.

The controller consequently cannot respond to disturbances in the grid (e.g. voltage breakdown) neither can it follow up- & down-loading of the turbine automatically.

The Manual Channel is for emergency or maintenance purpose. It has no automatic limiting characteristics (to keep within the allowed operating range)

as the auto channel does.

Summary

This section gave an overview of the generator fundamentals in such a way that the operating

range and limiting characteristics of the generator can be understood and applied in the field. Further it explained rudimentary the different types of Synchronous Generators and Excitation Systems.

To ensure that you understand the material covered, review each question in the Objectives

(page 4).


G

enerators Synchronous G

enerator:

Electrical Maintenance

Fundamentals &

Operation

AB

CD

Figure 1: Salient and Non-salient Pole G

enerator

S

N

90

Direct Axis Direct Axis

QuadratureAxis

QuadratureAxis

Rotor Winding

Rotor Pole

Air-Gap

Stator Conductors

Stator Core

Salient Pole Generator Non-Salient Pole GeneratorCSXA400353.cdr

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Figure 2: Synchronous Generators

Three phase synchronousHydrogenerator(salient poles)

Three phase synchronousTurbogenerator (air-cooled)(non-salient poles)

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Figure 3: Generation of a Single Phase Alternating Voltage

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U

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

0 30 60 90 120 150 180 210 240 270 300 330 360

Induced voltage in function of angle or timeMagnetic flux linked with the coil

u= U x sin

Coil

Magnet

Sliprings

U: Induced voltage

: Magnetic flux


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Figure 4: Principal Arrangem

ent of Stator Windings

CSXA400357.cdr

N

SX

n1

Tp180

U1 V1

U2

W1

23Tp

120

Tp3

N

S

n1n1phase 1

phase 2

phase 3

"Inner surface"of each loop are added together

Generation of a 3 phase voltageby the rotating rotor field

Sinusoidal field of rotor is movingrelative to the stationary stator winding(the curve is actually slightly stepped)

Figure 4.1

Figure 4.2

Rotorfield

Statorwindings

Equally drawn conductorsof stator belong to one phase

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Figure 5: Magnetic Field in a Generator at No-load Condition

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Field axisPole axis

Stator

Cooling holes

Rotor

Slots containingstator windings

Slots containingrotor windings

Rotation of rotor

Magnetic flux


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Figure 6: Magnetic Field in a G

enerator at Load Condition

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Up

U1

LoadangleRotor Field

Turbine/ Generator"pulling load behind"

Coupling like springor rubber band

Field axisLoad a

ngle

Pole axis

Figure 6.1

Figure 6.2

Armature Field or"Stator Field"

Grid / Load

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Figure 7: Synchronous Generator: Equivalent Circuit Diagram, Over-excited and Under-excited Vector Diagram

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Figure 7.1 Synchronous Generator: Equivalent Circuit Diagram

Figure 7.2 Over-excited Vector Diagram(with inductive load only)

Figure 7.3 Under-excited Vector Diagram(with capacitive load only)

( Generator working as capacity ) ( Generator working as inductivity )

X : Synchronous Reatanced X : Stray ReactanceX : Main Reactancehs

X = j ( X + X )d hs

Grid/ Load:inductive

capacitiveload

inductiveload

seen from the consumer side

Gen. produces reactive power (Q O)*U1I lagging1

Grid/ Load:capacitive

seen from the consumer side

*U1I leading1Gen. consumes reactive power (Q O)

Up

U1

I1

U = j X Id d 1

UpU1

I1

U = j X Id d 1

I1 j X Id 1

UdU1Up

I1 IfI

U1

Xs

Im

Xh IfI

If

RF UF

=^

*

Ud

UpU1* *

*

U1

I1

U1

I1

= +90

Ud

UpU1U1

I1

U1

I1

= -90


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Figure 8: Vector Diagram

and Power C

hart

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I1

I1

IfI

U1

X

Im

Xh IfI

If

Rf Uf

I

1

I1If Im= +

S = 1p.u.n

1Xd

*I =fo

I =foIfoI Im

N

Ud

U1Up

U = j X Id d 1

1Xd

1Xd

1Xd

I1

U1

UX

1d

= IfoI

I1

I = UXf

I p

dUp

U1

with U1 = 1 per unitI

I = field

curren

t

ffI

*Valid for a Turbogenerator)

P = active power

Q =reactive power

overexcitedlagging

at consumer side/gridI lags U

underexcitedleading

at consumer side/gridI leads U

no load field current

stator current

Rotation by -90

Figure 8.2Figure 8.1

(p.u.)

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Figure 9: Open Circuit and Short Circuit Curves

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A

F

G

H

D

p.u.

1.50

1.00

0.50

0.00 0.50 1.00 1.50 2.00B C

K K1

Rating of a typical turbogenerator

Effective powerAppearent powerStator voltageStator currentNo load field current

Stator armature leakage reactance

Unsaturated synchronous reactance

Saturated synchronous reactance

No- load field current

Short-circuit field current

Short-circuit ratio

500 MW588 MVA22'000 V = 1 p.u.15'440 A = 1 p.u.1'550 A = 1 p.u.

xa = 0.195 p.u.AD

= 2.450 p.u.ABAD

= 2.080 p.u.AC

AC= 0.481AC

AC = 1550 AMPS

AD = 3220 AMPS

Field current

Stator voltageStator current

Short c

ircuit cu

rve

Open circuit curv

e

Satur

ation

line

Gap l

ine

p.u.


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Figure 10: Power C

hart of a Turbogenerator: Operating R

anges and Limits

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0.5

0.5

0

A

CD

F

-1

1.0 p.u.

1 p.u.1Xd

If*=Eph

I f

X d

I g

E0.5

0.5

0

A

CD

F

PwIaw

1.0 p.u.

PaIaq1 p.u.1

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f n

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E

IfI

Ig

P

w

:

A

c

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p

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e

r

I

:

A

c

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v

e

c

u

r

e

n

t

a

w

Practical staticstability limit

Theoretical staticstability limit

Limit of stator current

Limit of rotor current

Pa : Reactive powerI : Reactive currentaq

: Load angle

: Phase angle

: Power factor (=cos ): Synchronous reactance

Xd

PF

: Field current

: Nominal field current

: Stator current

: Nominal generator current

If

Ifn

Ig

Ig n

Maximum turbinepower

" underexcited"Leading Power Factor (PF O)

" overexcited"Lagging Power Factor (PF O)

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Figure 11: Brushless and Static Excitation Systems

AVR

G3~

AVR

Rectifier

Gen

erat

orci

rcui

t bre

aker

Gen

erat

orci

rcui

t bre

aker

Rectifier

Rotatingrectifier

Fieldbreaker

Automaticvoltage regulator

Automaticvoltage regulator

Excitationtransformer

Excitationtransformer

Static Excitation System

Brushless Excitation System

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Figure 12: Major C

omponents of a Typical B

rushless Excitation System

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Pilotexciter

VoltageSensor

ACGeneratoroutput

Automaticor manual

Generator

Generator field orrotor winding

Turbine

DC AC

Rotatingrectifier Field windings

Main exciterPermanentmagnets

DC

3ph- AC supply

AVR

3~ AC

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Module ObjectivesIntroductionGenerator Types

Generator FundamentalsConductor in a Magnetic FieldSynchronous GeneratorReminder

OperationOperating Range - Power ChartOther Generator Charts

Generator ExcitationBrushless ExcitationStatic Excitation

Automatic Voltage Regulator (AVR)SummaryFigure 1: Salient and Non-salient Pole GeneratorFigure 2: Synchronous GeneratorsFigure 3: Generation of a Single Phase Alternating VoltageFigure 4: Principal Arrangement of Stator WindingsFigure 5: Magnetic Field in a Generator at No-load ConditionFigure 6: Magnetic Field in a Generator at Load ConditionFigure 7: Synchronous Generator: Equivalent Circuit Diagram, Over-excited and Under-excited Vector DiagramFigure 8: Vector Diagram and Power ChartFigure 9: Open Circuit and Short Circuit CurvesFigure 10: Power Chart of a Turbogenerator: Operating Ranges and LimitsFigure 11: Brushless and Static Excitation SystemsFigure 12: Major Components of a Typical Brushless Excitation System

Documents

Synchronous Generator Fundamentals and Operation