9-Generator Performance Curves and Static Exciation

7/29/2019 9-Generator Performance Curves and Static Exciation

1/141

Doc. n 196 W 429 Rev.1 Oct. 2008 1

AnsaldoEnergia Una Societ Finmeccanica

.

DPT. ELECTRICAL MACHINES

_____________________________________

PRESENTATION

PERFORMANCES CURVES OF THE ALTERNATOR

+

STATIC EXCITATION SYSTEM

_____________________________________


2/141

Doc. n 196 W 429 Rev.1 Oct. 2008 2


Summary

1st PART (sh. 3)

PERFORMANCES CURVES OF THE ALTERNATOR

2nd PART (sh. 39)STATIC EXCITATION SYSTEM

.


3/141

Doc. n 196 W 429 Rev.1 Oct. 2008 3

1st PART



4/141

Doc. n 196 W 429 Rev.1 Oct. 2008 4

PERFORMANCES CURVES

OF THE ALTERNATOR



5/141

Doc. n 196 W 429 Rev.1 Oct. 2008 5

The performances given by a generation groupof electric energy (type turbogenerator,hydrogenerator, etc.) they are essentiallysynthesized by the following types of curves,that represent an useful tool to support theexercise and maintenance operators of a

production plant.

1a - Saturation and short circuit curves

1b - Capability diagram

1c - V curve

1d - Power vs. temperature curve (for GT)



6/141

Doc. n 196 W 429 Rev.1 Oct. 2008 6

1 a SATURATION AND SHORT

CIRCUIT CURVES


G

T

25C

Iexc

nom

Vstator

Main Breaker

OPEN

At no-load

At nominal

speed

Temperature

The saturation curve of analternator shows the behavior of

the stator voltage versus the

variations of the excitation

current Iexc injected in the rotorwindings.

This curve is done at no-load

conditions, at nominal speed

revolution and at a defined

ambient temperature.


7/141

Doc. n 196 W 429 Rev.1 Oct. 2008 7

( 1 a Saturation and

short circuit curves )


The a) diagram shows that,

when the excitation current

increases, after a certain linear

trait, appears a knee of the

voltage V due to a saturationphenomenon of the magnetic-

iron material of which the

laminated core of the stator is

composed.

a)

Vstator

Iexc


8/141

Doc. n 196 W 429 Rev.1 Oct. 2008 8

The diagram also showsthe short circuit straight

line b) obtained

measuring the statorcurrent versus variations

of excitation current,

when the main terminals

of the stator have been

short circuited.


( 1 a Saturation and

short circuit curves )

b)

Iexc

Istator


9/141


10/141

Doc. n 196 W 429 Rev.1 Oct. 2008 10

1 b CAPABILITY DIAGRAM


It is a particularly important diagram because it

synthetically represents by a close surface all the

possible working points of the generation group

(turbine + alternator) in terms of MW and MVAR

delivered for the external loads.

This diagram therefore is valid if the generator is on

load only [i.e. when it is synchronised to the grid or

it is loaded locally (in island)].


11/141

Doc. n 196 W 429 Rev.1 Oct. 2008 11

( 1 b Capability diagram )


MW

+

-

MVAR

Over

Under

Mw are in abscissa and MVAR are in

ordinate.

Each segment of the star having origininto zero is composed by working points

having same ratio between MW-MVAR

and therefore the same cos .

Every half-circumference with center in theorigin is composed by working points at

constant MVA, but with different cos .

The curves are at Vstator= constant.

In this case the capability are three and

each of them corresponds to a generator

cooling more or less intense (here done

with hydrogen at different pressures).


12/141

Doc. n 196 W 429 Rev.1 Oct. 2008 12

The overexcitation quadrant is atpositive MVAR (positive cos).

The underexcitation one is at

negative MVAR (negative cos).

In the example at side, the nominalworking point (on which the whole

group is sized) is at cos= + 0,85

in overexcitation.

The boundaries of the curves are

due to the limits of the machine

project.



Nominal

working point

+

-

MVAR

Over

Under

MW


13/141

Doc. n 196 W 429 Rev.1 Oct. 2008 13

At side, the boundaries of the

capability, imposed by the sizingand by the characteristics of themachine, are highlighted withcolored lines.

More in detail:

green line: limitation due to themax thermal ability of the stator

blue line: limitation due to themax thermal ability of the rotor

red line: limitations due toproblems of dynamic stability andto heating of parts situated in theextreme zones of the statorpackage.



Nominal

working point

+

-

MVAR

Over

Under

MW


14/141

Doc. n 196 W 429 Rev.1 Oct. 2008 14

This curve is particularly meaningful for the exerciseoperators and therefore it deserves further details in

order to explain the manner by which it is gotten and

the reasons for which some limitations are applied to it.

The following images go through the sequence of all thephases of the capability construction and they focus,

particularly, the phenomena that converge to establish

its boundaries in the plan of the active (MW+) and

reactive (MVAR +/-) powers.


( Appendix to the item 1b )


15/141

Doc. n 196 W 429 Rev.1 Oct. 2008 15

AnsaldoEnergia Una Societ Finmeccanica( Appendix to the item 1b )

.

Now we go back through the notes on the capability already seen, starting from the

origin and using the physical considerations that have contributed to produce it..

------------------------------The capability diagram is represented on the Active/Reactive

Powers and it includes all possible working points of the

generator (each point is defined by its active/reactive values).

It is applicable to thesynchronized machine only and it dependsby the size of turbine and alternator,

Some limitations are applicable due to the materials temperatures

and to the dynamic performances of the group (stability).

The following images show in sequence the main considerationsthat produce, as final result, the definition of the capability.


16/141

Doc. n 196 W 429 Rev.1 Oct. 2008 16


.

1 step (pls. see next image)

On the surface Active/Reactive Power the portion at MW < 0

is not considered [because we are talking of generators MWdelivered to the grid(MW with sign > 0) and not motors

MW absorbed from the grid(MW with sign < 0)].

The size imposed by the Client (apparent power MVA) defines

on this plan an half-circle including all working points having

MVA the size required (its limit is just the half-circumferenceat max MVA).

The nominal cos required, together with the apparent powerrequired, define the nominal working point of the whole

group turbine-generator.



17/141

Doc. n 196 W 429 Rev.1 Oct. 2008 17


.


Nominal apparent power

(MVA) of the alternator

Active Power

Positive reactive

power

Negative reactive

powerUnder-

excitationLEADING

Over-

excitationLAGGING

0

Motor

Nominal working point

(nominal power at nominalcos )

(+)

(

-)

Generic working point

(MVA corrispondent to a

generic MW / MVAR)

MVAR

MW

MVA nominal

MVA

Doc. n 196 W 429 Rev.1 Oct. 2008


18/141

Doc. n 196 W 429 Rev.1 Oct. 2008 18

2 step (pls. see the following image)

The nominal working point (MVA, MW, MVAR, cos)is the main reference for turbine and generator sizing.

Consequently, the turbine is designed to deliver, as its maxpower, the MW correspondent to the nom. working point.

This is the first limitation that cuts the upper part of thecapability diagram.

It will be physically impossible to over-exceed that MWand, therefore, the upper highlighted area will be erased

because inaccessible (forbidden working conditions of thegroup).



19/141


20/141

Doc. n 196 W 429 Rev.1 Oct. 2008 20

3 STEP (pls. see the following image)

The nominal working point (MVA, MW, MVAR, cos)is the main reference for turbine and generator sizing.

The generator will have the stator size designed on theMVA of the nominal working point and, similarly, therotor size.

The outlined half-circumference (MVA=constant)represents all points at the same MVA of the nominal

working point.The rotor is sized for the steady field current correspondentto the nominal working point and this size limits theworking conditions up to the points correspondent to the

red line.Consequently, the blue area also (at right end of diagram)is cutted and inaccessible, because of the rotor size.



21/141

Doc. n 196 W 429 Rev.1 Oct. 2008 21

.

.



Active powerNominal apparent power

(MVA) of the alternator

and stator size (+)

Inaccessible area due to the

rotor size of the generator

Limitation due to

the rotor size

0

Positive reactive

power

Negative reactive

powerUnder-

excitationLEADING

Over-

excitationLAGGINGMotor

(

-)

Doc. n 196 W 429 Rev.0 March 2005



22/141

Doc. n 196 W 429 Rev.1 Oct. 2008 22

Just for example, eacharc of circumference inblue in the figure,

corresponds to workingpoints having all thesame excitation current.

In particular, the design ofthe rotor is sized on theexcitation currentcorresponding to thenominal working point Pn.

The center of these arcsis found in a specific pointsituated on the MVAR

axis in underexcitation atabout 1/Xd from thezero.


MW

MVAR

0

Nominal workingpoint Pn

1 / Xd

Overexc.

Underexc.



23/141

Doc. n 196 W 429 Rev.1 Oct. 2008 23

4 STEP (pls. see the following image)

The last limitation on the capability diagram (the blue

area at left end) is due to stability problems of thegenerator and to overheating of some parts at the statorcore ends.

The risk of instability is mainly due to weak flux inside the

machine air-gap that makes feeble the magnetic connectionbetween rotor and stator, with consequent its easy tear.

The overheating on the extreme parts of the statorwindings, due to a particular magnetic flux disposition in

that zone, is the second reason of limitation on this under-excitation area of the capability diagram.



24/141

Doc. n 196 W 429 Rev.1 Oct. 2008 24

.


Inaccessible area due to stability

problems and overheating of

some parts at the stator core ends.

Active power

MW

0

Positive reactivepower MVAR

Negative reactivepower MVAR

Under-excitation

LEADING

Over-excitation

LAGGINGMotor

(-)

(+)

Limitation

Nominal working pointHeating

problems

Stability problems



25/141

Doc. n 196 W 429 Rev.1 Oct. 2008 25

The final configuration of the capability diagram for theturbo-generator set is shown on the following image.

All permissible working points (at steady state) for the group

( if synchronized ) are included into the green area only.

The protection to avoid the MW over-exceeding (to shift toordinates higher than the max allowed) is intrinsic due tothe turbine size.

On the contrary, the protections against working attempts

out of the above mentioned limitations at right or left ends,are foreseen and performed by the excitation system.



26/141

Doc. n 196 W 429 Rev.1 Oct. 2008 26

Capability diagram of generator


Inaccessible area due to the

turbine power limitation

Inaccessible area due to therotor size of the generator

Inaccessible area due to stability

problems and overheating of some

part at the stator core ends.

Max apparent power of the

generator

Active power

Positive reactive

power

Negative reactive power Under-excitation

LEADING

Over-excitation

LAGGING

0

Motor

Under-excitation limit Max rotor current limit

Max turbine

power



27/141

Doc. n 196 W 429 Rev.1 Oct. 2008 27

CAPABILITY DIAGRAM OF GENERATOR

Movements of the working points


MW

MVAR

Over-excitation

Under-excitation

AVR

AVR

EHC EHC

Zone

MOTOR


From the point of view of the machineconduction, we remember that any

change of the working point on the

capability is done by commands on

the EHC (turbine regulation) and/or on

the AVR (voltage regulation into theexciter).

In particular, as shown at side, the

variations of MW are obtained by

actions on the EHC only, while the

variations of MVAR are produced by

commands on the AVRonly.


28/141


29/141


30/141


31/141

Doc. n 196 W 429 Rev.1 Oct. 2008 31

AnsaldoEnergia Una Societ Finmeccanica.

V curve of the generator

I excitation

P apparent

0

Nominal

working point

P app nom

Iexc at no-loadA

B

E

I exc nom

Limitation dueto the max

power of theturbine

.% Iexc

at no-load

D C

OverUnder

( 1 c V curve )

The following image shows the correspondence of some working points

both on the capability diagram and on the V curve.


32/141

Doc. n 196 W 429 Rev.1 Oct. 2008 32


.

( 1 c V curve )

Correspondence between Capability and V curves


33/141


34/141

Doc. n 196 W 429 Rev.1 Oct. 2008 34


.

MW

MVAR

cos = 0.85

MVA

0

Sovraecc.

Sottoecc.

( 1 d Power vs. temperature )


35/141

Doc. n 196 W 429 Rev.1 Oct. 2008 35

GENERATOR SYNCHRONIZED :

OVER-EXCITATION &

UNDER-EXCITATION



36/141

Doc. n 196 W 429 Rev.1 Oct. 2008 36


The image at side represents, symbolically and from the reactive

power point of view only, the generator synchronized on the

national grid.

GX

ReteVa Vr

The interconnecting reactance X

allows the reactive power exchange

between generator and grid.

In the synchronizing instant Va=Vr : no flux of reactive current

through the reactance is there ( I = (Va-Vr) / X ---> I = 0 = Q ).

If Vr remains constant and Va changes, reactive current increases.

If Va > Vr I and Q go from G to grid (over-excitation).

If Va < Vr I and Q go from grid to G (under-excitation).


37/141

Doc. n 196 W 429 Rev.1 Oct. 2008 37


In over-excitation Ia flows from G to the grid and produces on X a

voltage drop with sign + at G side.

In under-excitation, on the contrary, Ia flows from the grid to G

and produces on X a drop voltage with sign + at grid side.

Being Ia always 90 delaied on Vt, in over-excitation the Va-Ia

vectorial diagram says that G sees an inductive load, while in

under-excitation G sees on the contrary a capacitive load (pls.see this images).

GX

GridVa Vr

Ia

Vt

Over-excitation

+_

GX

GridVa Vr

Ia

Vt

Under-excitation

+_

Inductive load Capacitive load


38/141


39/141

Doc. n 196 W 429 Rev.1 Oct. 2008 39


2nd PART


40/141


41/141


42/141


43/141


44/141

Doc. n 196 W 429 Rev.1 Oct. 2008 44

When the generator is synchronized, the grid creates on its

stator winding a strong rotating electromagnetic field (like

magnet), which finds in front the rotor magnet with correct

polarities and their attraction is consequent.

But their magnetic connection is strictly dependent by the air

gap magnetic flux, which is strong if the rotor magnet is

powerful and fragile if the rotor magnet is feeble.The energy exchange between generator and grid is controlled

through the regulation of that flux.

Consequently, a rotating permanent magnet could not control

that exchange, while a rotating magnet produced by field

excitation (d.c. current) can properly regulate that exchange.



45/141


46/141


47/141

Doc. n 196 W 429 Rev.1 Oct. 2008 47

A machine is ageneratorif its rotor magnet, powered by

the mechanical couple of the turbine, rotates ahead of the

stator magnet (of the grid) and pulls and drags it.

On the contrary, a machine is a motorif the stator magnet

(of the grid) rotates ahead and pulls and drags the

rotor magnet.

The angle displacement between the axes of the rotor and

stator magnets is called load angle.

We are always speaking aboutgenerators .




48/141

Doc. n 196 W 429 Rev.1 Oct. 2008 48

In order to deliver big amounts of energy to the grid, the

turbine gives high torque couples to the generator shaft.

But that high couples, applied to the rotor body/magnet,

succeed to drag the stator magnet if only the magnetic flux

inside the air gap is very strong ( rotor-stator link, in thiscase, must be very solid to do it). Strong flux means

high intensities of excitation current on the rotor winding

(this is the over-excitation status).

On the contrary, when the excitation current is reduced, thepossibility to transmit energy to the grid is correspondingly

reduced because the flux is reduced.

In that conditions it is hazardous have strong couples onthe shaft because it becomes easier to tear the magnetic

link between rotor and stator magnets (under-excitation

status).



49/141

Doc. n 196 W 429 Rev.1 Oct. 2008 49

The previous general introduction clarifies the necessity

and the advantages to create an artificial magnet on the

rotor through d.c. currents flowing into windings mountedon the rotor body.

The excitation systems was conceived just with the main

purpose to supply d.c. currents for the rotor windings.

Nevertheless this is not the unique goal of the exciters and,

taking advantage of the control facilities on the fieldcurrent, other important functions can be performed.



50/141


51/141


52/141

Doc. n 196 W 429 Rev.1 Oct. 2008 52

.


+_

Alternator

Rotor rings

Dynamo

+

_

Alternator and

rectifier bridge(the electrical scheme

is for brushless too)

+

_

+

_

Static

Various types of

excitation system



53/141

Doc. n 196 W 429 Rev.1 Oct. 2008 53

ADVANTAGES OF FULLY STATIC EXCITER______________________

HIGH RELIABILITY SHORT TIMES TO RESTORE EVENTUAL FAULTS(due to the static components only) (economic advantages for reduction of production loss)

COMPLETE REDUNDANCY SIMPLIFIED MAINTENANCE

(ex. brushless is not doubled) (practically nothing)

DYNAMIC RESPONSE FAST FIELD DE-EXCITATION

(negligible delays and negative ceiling) (direct on the machine field)

GLOBAL EFFICIENCY SHORTER LENGTH OF THE SHAFT

(reduced losses) (reduced dimensions)

For all that reasons the plant strongly gains on its economical balance.



54/141


55/141



56/141

Doc. n 196 W 429 Rev.1 Oct. 2008 56

+

EXCITATION

CUBICLE

MAIN

GENERATOR

EXCITATION

TRANSFORMER

ENERGY FOR

EXCITATION

SYSTEM

STATIC EXCITATION SYSTEMSTATIC EXCITATION SYSTEM

ENERGY FORTHE ROTOR

WINDING




57/141

Doc. n 196 W 429 Rev.1 Oct. 2008 57

Typical parameters for evaluation

of the excitation systems performances______________________

CEILING

REGULATION ACCURACY

SYSTEM RESPONSE

SYSTEM SETTLING TIME

g


58/141


59/141



60/141

Doc. n 196 W 429 Rev.1 Oct. 2008 60

.

t [sec]

V stat. generat.

The regulation accuracy is evaluated by the displacement, at steady, between

Vg2 (final value after oscillation) and Vg1 (previous value).

Vg2Vg1

ACCURACY

displacement

O



61/141

Doc. n 196 W 429 Rev.1 Oct. 2008 61

RESPONSE

It is a parameter that gives the idea of the promptness by which theexcitation system responds to the variations, mainly in automatic

regulation mode.The international Standards define this evaluation method asshown into the following image (find the compensation line, markthe intersection point at 0.5s and apply the indicated formula).

Starting from the nominal excitation voltage, the ceiling voltage isimposed by the regulator and the resulting excitation voltage shapeon the field is a curve significant of the exciter response.

The response measure has meaning for rotating exciters onlybecause for the static exciters it is directly proportional to the

ceiling.

g



62/141

Doc. n 196 W 429 Rev.1 Oct. 2008 62

.

t [s]

V excitation

O

A

BC

V ceiling

A

R = AB / (BC*OC)R = AB / (BC*OC)

0.5 s

RESPONSE

V nominal excitation

Rotating exc. RED AREAS

Static exc. GREY AREAS

AC = compensation line

considering instant t=0.5 s, area ABC must be equal

to the area under the real curve (the exponential for

rotating exciter and the step for static)

01


63/141



64/141

Doc. n 196 W 429 Rev.1 Oct. 2008 63

.

t [sec]

V stat. generat.

Time t0 is defined as the interval from t=0 to the instant in which the oscillation returns and

remains contained into a certain band up to the steady state.

The width of this band (. %) has to be defined in order to be able consequently to define t0 .

Vg2

Vg1

t0

Band ( . %)

SETTLING TIME

O



65/141

Doc. n 196 W 429 Rev.1 Oct. 2008 64

STATIC EXCITER

g



66/141

Doc. n 196 W 429 Rev.1 Oct. 2008 65

.

STATIC EXCITATION SYSTEMONE LINE BLOCK DIAGRAM

STATIC EXCITATION SYSTEMONE LINE BLOCK DIAGRAM

G

AVR 1

AVR 2

CB

RES

EXCITATION

TRANSFORMER

Ref. 1

2

EXCITATION

BOARD

MAIN

GENERATOR

Commands

and signals

from remote

Bridge 1

Bridge 2

+

_

Signals

to remote


.


67/141

Doc. n 196 W 429 Rev.1 Oct. 2008 66

g

TYPICAL INTERFACE OF THE EXCITATION BOARD

INPUT OUTPUT

Excitation

board

3 LV power line

from excitation transformer

Analogical signals from remote

Commands from remote

Permissives from remote

Trip from remote

d.c. power output (+ and -)

to the rotor of the machine

Analogical signals to remote

State signals to remote

Alarms to remote

Trip request to remote

Aux feeding voltages


68/141


69/141


.


70/141

Doc. n 196 W 429 Rev.1 Oct. 2008 69

A particular feeding system taken from the generator terminals, now nomore used, is with a voltage transformer (TRE) and a current transformer

(TAT). The TRE feeds a thyristors bridge which is in series with a diodes

bridge fed, on the contrary, by the TAT. In this case too it is necessary the

use of the pre-excitation circuit.

When the generator is at no-load, the

machine is excited by the contribution of

the TRE only (in fact the stator current

I=0). If the generator is in short circuit

(also permanent) it is the TAT alone that

contributes to excite it (in fact the stator

voltage V=0). In any other working

condition (between these two extreme)

both the transformers contribute to excite

the machine. The quick intervention on

the line faults of the actual digital

protections has made useless this systemwhose principal characteristic was just to

succeed in sustaining for long time the

short-circuits prolonged.

Static

exciterGen

Voltagetransformer(TRE)

Currenttransformerwith air-gap (TAT)

V of the

machine

I of the

machine

From

TRE

From

TAT

+

-

+

-

Thyristors

Diodes


71/141


.


72/141

Doc. n 196 W 429 Rev.1 Oct. 2008 71

UPS or plant Battery

Feeder

To regulation

system 1

To regulation

system 2

Feeder

Exciter

Each regulation system is fed with security and with the possibility to becompletely deactivated for eventual maintenances during the normal

service also, while the alternator produces energy under the control of the

other regulation system.

Since the feeders of the electronics (1 for regulator) can be fed both in

a.c. and in d.c., it can be chosen, as energy source for them, both the UPS

and the battery of the plant.



73/141

Doc. n 196 W 429 Rev.1 Oct. 2008 72

POWER SECTION

RECTIFIER BRIDGES



74/141

Doc. n 196 W 429 Rev.1 Oct. 2008 73

The 3phase power rectifier bridge Graetz type can be madeby diodes or by thyristors.

Diodes rectifier bridge .

Diode is a non-controlledsemiconductor

having 2 poles: anode and cathode.

Its conduction is conditioned by 1 status

only: potential of anode Va higher than the

potential of cathode Vc.

In this condition the current I can flow through the diode.

A Graetz rectifier bridge with 6 diodes (next image) has its

output directly proportional to its feeding voltage only.

+

_

I

Va

Vc

anode

cathode



75/141

Doc. n 196 W 429 Rev.1 Oct. 2008 74

DIODES BRIDGE

RS

T

+

--

i

i

iexc

VRVS

VT

Vexc

Time

Vexc

VR VT

VR

VS

VT

VS

VR

V

RV

TV

S

VRS VRT VST VSR VTR VTS

1 period of the fundamental

[ 20 ms (50Hz) 16.6 ms (60Hz) ]

The output excitation voltage

Vexc (left side) is, in anyinstant, the highest amongthe actual differencesamong the 3 phase voltagesVR - VS - VT at that instant(phase-to-phase values).The result is a regular

sequence of the 6 typicalondulating peaks duringeach 20 ms (cycle 50Hz).Its average value is directlyproportional to VR VS VTfeeding values only.

V

R

V

S

V

T

V

R

V

S

V

T

V

R

V

S

V

T

R+

R -

S+

S -

T+

T -

Phase voltages

Vexc


76/141



77/141

Doc. n 196 W 429 Rev.1 Oct. 2008 76

THYRISTORS BRIDGE

The output average value of Vexc isnot directly proportional to the supplyvoltages VR VS VT , as per the diodes

bridge, but it is function of thecontrolling signal on the gate of thethyristors.Changing the delay of their firing

instant (changing the firing angle )referred to the natural firing instant, itis possible to change the output bridge.

This kind of bridge has the possibility toproduce a rectified voltage withvariable average value, being constantits feeding voltage.Theoretically, changing the control, theoutput voltage Vexc could be changed

from + VC to VC , where Vc is theceiling voltage (posit.), correspondent

to the output voltage with = 0 (whichis the condition where the thyristorsbridge becomes like a diodes bridge).

RS

T

+

--

i

i

iecc

VR

VS

VT

Vecc

R+ S+ T+

R - S - T -

R+ means thyristor of the

phase R, sidepositive (+)

Vexc

Time

- VCeiling (=180)

0

Permanent times( Vexc > 0 )Voutput

(average value)

Zone used for

transient timesonly ( Vexc < 0 )

+ VCeiling

(=0)



78/141

Doc. n 196 W 429 Rev.1 Oct. 2008 77

THYRISTORS BRIDGE

RS

T

+iecc

VRVS

VT

Vecc

R+ S+ T+

R - S - T -

--

R+

S+

T+

R -

S -

T -

60 60 60 60 60 60

R+S- R+T- S+T- S+R- T+R- T+S-

In the thyristors bridges, the

sequence of the commutations is

identical to that of the diodes

bridges (pls. see the image).

They are :

R+ S- for 60 electric

R+ T- for 60 electric

S+ T- for 60 electric

S+ R- for 60 electric

T+ R- for 60 electric

T+ S- for 60 electric

Each of the 6 thyristors conducts

for 120 electric, in couplealternatively with the two

thyristors of opposite sign of the

other phases.


79/141



80/141

Doc. n 196 W 429 Rev.1 Oct. 2008 79

THYRISTORS BRIDGE

Delay angle 0

=180

60 60 60 60 60

R+ R-T- S+ T+ S-

Delay angle =180

R+ R-T- S+ T+ S-

60 60 60 60 60

Average = 0

Working conditions with 90 ,and consequently with average voltage negative,

can be for transient times only

Averagenegative

Time

Time

Vexc

Vexc

Natural

instant of firing

of the thyristor

R+

They move

Bridge voltage

output for = 90

Bridge voltageoutput for = 140They move

0

Delay

on R+= 90

Delay

on R+= 140



81/141

Doc. n 196 W 429 Rev.1 Oct. 2008 80

The thyristors bridge has the particular possibility to deliveroutput voltages with negative sign, even if the semiconductors arecontrolled diodes (unidirectional).

This is possible because the bridge feeds a big inductive load

(field of generator) and any voltage variation on it produceschanges in current very slow, compared with the voltage changes(inside an inductance any current variation is braked).

Consequently, with the thyristors impulses delayed at >90, the

voltage transmitted to the field is negative but the thyristorsremain alive and in conduction until the current inside remainsalways positive (current is decreasing because of the V< 0).

If the current would reach zero the thyristors and the bridge wouldbe switched-off : this is why the time to deliver V< 0 on theoutput bridge is limitedby the time of decreasing to zero of thecurrent.



82/141

Doc. n 196 W 429 Rev.1 Oct. 2008 81

The static exciter are normally provided with twoidentical Graetz thyristors rectifier bridgesworking in alternative.

Each of them can be commanded by its own AVRonly or by both AVRs, operating in alternative.

Any malfunction of the main bridge produce theautomatic change-over to the other, in order toguarantee the continuity of the generator service.

The cooling of the rectifier bridge is normallydone by forced air in open cycle, except for the

exciters at small sizes (natural air) or at big sizes(treated water in closed cycle).



83/141

Doc. n 196 W 429 Rev.1 Oct. 2008 82

EXAMPLE OF CONVERTER COOLEDBY AIR IN OPEN CYCLE

3phase a.c. line from

excitation transformer

2phase d.c. line for therotor of the alternator

3phase Graetz bridge

with thyristors and fuses

3phase Graetz bridge

with thyristors and fuses

Warm air

Air

filters

Air

filtersFresh air



84/141

Doc. n 196 W 429 Rev.1 Oct. 2008 83

EXAMPLE OF CONVERTERS COOLEDBY AIR IN OPEN CYCLE

Fresh

air

Warm air

Hot

components

Lateral section of the cubicle

Path of the air that

cools a

compartment by

natural convection.

Filters

Path of the air that

cools a

compartment by

forced cooling.

Fresh

air

Warm

air

Hot

components

Lateral section of the cubicle

Extractor

Filters



85/141

Doc. n 196 W 429 Rev.1 Oct. 2008 84

EXAMPLE OF CONVERTERS COOLED

BY AIR IN CLOSED CYCLE

Air-to-water exchangerWater

Fresh air

Warm air

Extractor

Bridge

Sealed

room



86/141

Doc. n 196 W 429 Rev.1 Oct. 2008 85

EXEMPLE OF CONVERTER COOLED BY WATER

Schematic representation of the converter and its cooling system done

with treated water in closed loop

(in the figure, the representation of the water that flows in the bridge,

obviously, is indicative only, for explanatory purpose)

FUS

+

Cooling water coming from an

external hydraulic circuit for the

internal water-to-water exchangers

Ex Ex

P

Tk

P

T

P = circulation pump

Ex = water-to-water exchangerTk= treated water tank

T = cells of water treatment

FUS= fuses of the bridge

3phase Graetz bridgewith 12 thyristors

(100% series redundance)and 3 fuses at input side

Excitation board



87/141

Doc. n 196 W 429 Rev.1 Oct. 2008 86

DE - EXCITATION

AND

PRE - EXCITATION



88/141

Doc. n 196 W 429 Rev.1 Oct. 2008 87

The de-excitation of the generator reduces its stator voltage

at about zero (except the effect of the magnetic residual).

This condition is obtained with the discharge of the internal

flux, dissipating the field current through a passiveresistance circuit, composed by the rotor resistance and an

aux discharge resistance connected in series.

This is obtained by the crow bar positive (pls. see image)which can be fired by the logic with two voltage thresholds:

higher used when CB is operated as overvoltage protection

and lower used only when CB is actuated as field breaker.

Its ON state allows the field current flow through RES andthe consequent electromagnetic flux discharge.

Crow bar positive



89/141

Doc. n 196 W 429 Rev.1 Oct. 2008 88

.

Crow-bar positivefiring voltage 1 threshold

Overvoltage

protection

2 threshold

De-excitation

function0 Volt

Iexc

Rotor

Field

resistanceDischarge

resistor

Crow-bar

positive

Power

converter

De-excitation circuit or

Static field breaker

RES

0 time

Stator voltage

Being RES about double of field

resistance, the complete discharging

time is approx. equivalent to Tdo.

= Lfield / (RES + Rfield) = Tdo / 3

Tdo = Lfield /Rfield

CROW BAR



90/141

Doc. n 196 W 429 Rev.1 Oct. 2008 89

CROW - BARThe crow-bar is made by 2 thyristors in antiparallel (v. figure), one called positive crow-bar and the

other negative crow-bar.

Both has the function of protection against the overvoltages (+ and -) but the positive one has the

function also to de-excite the field of the alternator.

Its two functions are discriminated by two different thresholds of intervention that are him imposedaccording to the situations and of the moments in which it has to intervene (v. figure).

The thyristors firing is produced by the same overvoltages that directly activate the firing circuits,

which are totally redounded, to guarantee the certainty of the primer.

Crow-bar

positive

Crow-bar

negative

Dischargeresistor

Iexc

Iexc

+

_

CB

RotorIexc

RES

Primer thresholds of

the positive crow-bar 1 threshold

Protection

against the

overvoltages

2 thresholdFunction of

de-excitation0 Volt

Mechanism of de excitation by crow bar



91/141

Doc. n 196 W 429 Rev.1 Oct. 2008 90

Mechanism of de-excitation by crow-bar

IexcRotor

Dischargeresistor

Crow-bar

positive

Power converter

De-excitation circuit or

Static field breaker

Excitation voltage (for example) before

the de-excitation command

Negative ceiling

voltage

Instant where the de-excitation

order is done and it is ordered

the negative ceiling

Voltage on the

discherge resistor

and on the field

Instant where the output voltage of the bridge

equalizes the voltage on the discherge resistor.

After this point the bridge goes OFF because its

thyristors become inversely polarized

time

Voltage

0

Instant of command

of the impulses

suppression

Phase-to-phase

voltage

It is shown the mechanismby which the de-excitation ofthe generator is done by thecrow-bar.

PRE EXCITATION



92/141

Doc. n 196 W 429 Rev.1 Oct. 2008 91

PRE-EXCITATION

To the generator field

Pre-excitation circuit

Feeding from the

battery of the plant

+

_

Res+

_

To the generator field

Pre-excitation circuit

Feeding from an a.c.

aux line of the plant

+

_

When the feeding of the power converter is directly absorbed from themain terminals of the generator, at any starting it is always temporarilyactuated a dedicated pre-excitation system in order to give a first voltageramp sufficient to make the group independent from its own excitation.

The feeding source for the pre-excitation circuit can be a 3phase aux lineof the plant or the d.c. services (battery system) of the plant.

A dedicated logic automatically activates and deactivates this circuit andcontemporarily oversees the correct behaviour of this phase.

.


During the pre-excitation phase the feeding(f l b tt ) d


93/141

Doc. n 196 W 429 Rev.1 Oct. 2008 92

Excitation

transformer

Generator

Main breaker

of the machine

Rotor

Stator

Bloc diode

Battery of

the plant Circuit of

pre-excitation

Resistor of adaptation

and limitation

+

_

Exciter

Generator circuitry during the pre-excitation

0

time

Iexc

Tdo

Typical time

of the pre-

excitation

Max time allowed

to the pre-excitation

30% Iexc at no-load

During the pre excitation phase the feedingsource (for example battery) produces anexcitation current having an exponentialshape, with a steady state value of about 30%of the machine field current at no-load and atime constant equal to about Tdo of thegenerator.

Typical duration of the pre-excitation is someseconds (pls. see the diagram at bottom).

In case of problems, the logic of overseeingasks for the electric trip if the max timeallowed to this phase is expired (this time is

previously adjusted into the system).



94/141

Doc. n 196 W 429 Rev.1 Oct. 2008 93

PROTECTION CIRCUITS

PASSIVE PROTECTIONS ON THE POWER CIRCUITS



95/141

Doc. n 196 W 429 Rev.1 Oct. 2008 94

PASSIVE PROTECTIONS ON THE POWER CIRCUITS

Fuses

Excitation

transformer

Voltage limiters

Bridge

+

_

The exciter foresees two types of passive protections on the power circuits: fuses in

series to each thyristor and voltage limiters on the 3phase feeding line of the power

converter.

The fuses, obviously, protect the corresponding thyristors, having their I2t lower than the

I2t of the correspondent semiconductors.

The limiters (i.e. varistors) are three, connected triangle, and they limit the possible

overvoltages on that 3phase feeding line.

PROTECTION AGAINST SHORT CIRCUIT ON THE D.C. LINE



96/141

Doc. n 196 W 429 Rev.1 Oct. 2008 95

A.C. Protection

+

_

Gen

Fuses

Crow-bar

Excitationtransformer

Rotor

RES

Rectifier bridge

Short

circuit

This electronic protection has the purpose to avoid damages to the powerconverter against occasional possible short-circuits on its output bars (alongthe whole two-phase line that connects the power converter to the rotor of themachine).

Its intervention consists of sending the immediate command to the firingcircuits of the thyristors for their max impulses delay and contemporarily toproduce the electric trip of the generator.

PROTECTION AGAINST CURRENT UNBALANCE A.C. SIDE


Thi l t i t ti h th t d t t ibl b l th


97/141

Doc. n 196 W 429 Rev.1 Oct. 2008 96

Anomalous current

due to the fault

The current in the central phase S is greater than the

current in T, because S feeds T and the fault in R too.

The currents in the phases S and T should be equal

but, in this case, this in not true and this is detected.

R

S

T

Iexc normal

Iexc normal

Rotor

This electronic protection has the purpose to detect possible unbalances among the

currents values flowing into the three conductors (R-S-T) of the a.c. line that feeds

the power converter.

Such condition can occur in case of an internal fault of the rectifier bridge.

Its intervention consists of actuating the logic of bridges commutation, in order to

try to maintain the generator in service through the use of the backup rectifier

bridge.

If this was not possible it would be required the electric trip of the generator.

PROTECTION AGAINST MAX OVERLOAD CURRENT



98/141

Doc. n 196 W 429 Rev.1 Oct. 2008 97

Example of 2 points on the

curve overload limitZone (green) of

overload allowed

by this protection

0

Iexc

time

t2

I2

I1

t1

Inom

This electronic protection has the purpose to avoid damages to the power

converter due to possible thermal overloads.

It allows the bridge to deliver also currents higher than the nominal, but only if

they are contained within a threshold-limit following the quadratic law at inversetime shown in figure (higher overcurrents for brief times and lower overcurrents

for longer times).

Its intervention produces the request of electric trip of the generator.

CURVE IMPLEMENTED

IN THE AVR



99/141

Doc. n 196 W 429 Rev.1 Oct. 2008 98

AUXILIARY ACCESSORIES OF

THE EXCITATION SYSTEM

ROTOR EARTH FAULT RELAYhi d i i i i f h



100/141

Doc. n 196 W 429 Rev.1 Oct. 2008 99

This device guarantees a continuous monitoring of the

insulation level of the rotor winding, referred to the earth

potential (normally the rotor body).

In normal conditions this insulation level must be very high

(M), while, if it drops (some K), it is significant of somefault into the rotor winding, which is generated by

insulation loss between the winding copper and the rotor

body (i.e. insulation material of the rotor winding damaged

in some point).

This kind of fault, if occurred in one point only of the rotor

winding, does not produce, from the electrical point of

view, any visible immediate effect, but always it stronglysuggests to stop the generator service as soon as possible, in

order to avoid any further fault in other different points.

In fact this situation could produce injurious effects to the

system.

This device can be chosen with one or two monitoring

thresholds on the rotor winding insulation level and thecorresponding operations are the following:

- with one threshold its action is an alarm only

- with two thresholds its actions are alarm (step 1) and trip

(step 2)

GEN

Rotor winding

Rotor earthfault relay

+

-

Eventual fault

ROTOR TEMPERATURE CALCULATOR



101/141

Doc. n 196 W 429 Rev.1 Oct. 2008 100

GEN

Rotor winding

+

-

Vexc

Iexc

Rotortemperature

calculator

Iexc Vexc

Excitation board

Output signal

for remote

C

This device guarantees a constant monitoring of the rotor winding temperature, during the exercise of the

generator.

Its working principle is based on the continuous measure of the excitation voltage and current in order to

calculate the actual temperature of the winding by the ratio Vexc/Iexc at any time interval of some

milliseconds (cycle time of the digital program).

Of course the function is programmed considering the high thermal inertia of the rotor winding, compared

with the variation times of the excitation voltage and current (i.e. the ceiling voltage impulses are notaffecting the calculations because too fast for a possible influence on the temperature winding).

This function is performed by a dedicated software (subroutine) implemented into the main AVR program.

The digital regulation system gives an output signal significant of the rotor winding temperature for remote

uses (typically for a recorder in control room or for the DCS system).



102/141

Doc. n 196 W 429 Rev.1 Oct. 2008 101

REGULATION

The generator is provided with a regulation system which is



103/141

Doc. n 196 W 429 Rev.1 Oct. 2008 102

The generator is provided with a regulation system which is

the main responsible for the correct conduction of the

machine.

This system primarily operates in automatic mode, in order tohave the best dynamic performances and securities, but it is

possible the operation in manual mode too (next image).

The first of the possible regulations is on the stator voltage,which normally has to be maintained constant or regulated

following particular logic useful for the plant exercise.

Other types of regulation can be done (i.e. cos of machine).The regulation system is digital and normally fully redundant,

in order to be classified as fault tolerant.

Automatic and Manual regulation modes



104/141

Doc. n 196 W 429 Rev.1 Oct. 2008 103

Automatic regulation

mode

GENRegulation

& Excitationsystems

Stator

voltageElectric

feed-back

Set-point

selection

Automatic

correctionManual regulationmode

GEN

Stator voltageindicator

Human visualfeed-back

Continuous human

correction

Regulation

& Excitationsystems

CLASSIC REGULATION LOOPOF THE STATOR VOLTAGE



105/141

Doc. n 196 W 429 Rev.1 Oct. 2008 104

OF THE STATOR VOLTAGE

GErr.

Feedback

Reference

VT feedback

Bridge

+

-

Exciter

Next image shows the equivalent internal circuit of generator. Itincludes an ideal generator E and its synchronous reactance Xd1,



106/141

Doc. n 196 W 429 Rev.1 Oct. 2008 105

g y d1,

and it is connected to the grid by Xe1 (external reactance).

The diagram shows the voltages E1-V1-VGRID in two situations

- at no-load, just synchronized ---> E10 = V10 = VGRID- loaded, delivering positive MVAR to the grid (inductive load)

In last case, supposing VGRID=constant, increasing field current Iexc ,

E1 increases too (E10 --> E1A) and consequently V1 too (V10 -->

V1A), delivering to the grid an inductive (+) reactive current I1 ,and a consequent power rate QA , proportional to the difference

V between the voltages (E1A-VGRID).

Being Xd1>>Xe1 , large part of that V is on Xd1 and, consequently,

V1 will be always very close to the VGRID (transferred to thegenerator side) : this is the reason way, when the generator is

synchronized, the stator voltage is blocked on the grid value.

.- Generator synchronized -Blocks representation

- Generator synchronized -Equivalent electrical

representation



107/141

Doc. n 196 W 429 Rev.1 Oct. 2008 106

VGRID

xe1

G

Generator

Stepuptransformer

iexc1E

Generator

xd1

xe1

iexc1

E1 V1I1

V GRID

G

Stepuptransformer

representation

Internal gen.voltage E1Terminals gen.

voltage V1 V GRID

AT NO LOAD : E10=V10=VGRID

Command to increase iexc1in order to change

from E10 (at no load)

to E1A

(Q (+) delivered)

V1A

V10V

GRIDE10

E1 A

QA

E1A

- VGRID

QA

E1A

- VGRID

xe1xd1

G

.


In case of increasing of the external reactance Xe of the transformer, from

Xe1 to Xe2 (pls. see diagram below), with same voltages E1 and VGRID, thereactive current exchanged Ireact and also Q decrease because they are

always proportional to the PENDENCY of the segment (E1-VGRID) - and not

- Generator synchronized -Equivalent electrical

representation


108/141

Doc. n 196 W 429 Rev.1 Oct. 2008 107

always proportional to thePENDENCYof the segment (E1 VGRID) and not

only to the level difference among these two voltages - according to the

relationship:

Ireact Q (E1-VGRID) / (Xd1+Xe)

which, being calculated as ratio among the 2 cathetus of the triangle

E1-E10-VGRID, it represents just the pendency of its hypotenuse, that is thependency of the aforesaid segment (E1-VGRID).

The pendency of (E1-VGRID 1) is greater of (E1-VGRID 2) therefore, with the

external reactance Xe2 > Xe1 Ireact/Q are smaller (smaller pendency).

E

Generator

xd1xe

iexc1

E1 V1I1

V GRID

G

Stepuptransformer

p

Internal volt.generatorE1

21

E10

E1

Ireact Q (E1 V

GRID) / (Xd1 + Xe)

Ireact Q (E1 VGRID) / (Xd1 + Xe)

xe1xd1

Inside the Generator

xe2unchanged but is Xe that changes

Xe2 > Xe1

Xd1 obviously remains

V GRID

The automatic regulation operates as explained in the followingimage.



109/141

Doc. n 196 W 429 Rev.1 Oct. 2008 108

If, for example, the grid voltage drops (VGRID A --> VGRID B), the

generator terminals voltage V1 drops too (V1A --> V1B) being

still unchanged the field current Iexc and the internal voltage E1A(pls. see the upper diagram of the image).

In that moment the delivered reactive power increases because

increases V between E1A and VGRID B .

The AVR sees V1 drop and corrects it increasing the field currentIexc [and consequently E1 (E1A --> E1C)] up to the condition in

which V1 come back from V1B to V1A (pls. see the second

diagram of the image).

Due to this, the reactive power increases more up to QC .

Same sequence, but inverted, if VGRID A jumps higher.

.


GRID VARIATION AND REGULATOR REACTION

GRID VARIATION AND REGULATOR REACTION

D lt th


110/141

Doc. n 196 W 429 Rev.1 Oct. 2008 109

Correction due to the

automatic regulation

to bring back V1

from V1B to V1A( which is the original

value )

Internal gen.voltage E1

Terminals gen.voltage V1 V GRID

V1AE1 A

xe1

xd1

Inside the Generator V1B

VGRID B

QC E1C - VGRID B

IC ,QC > IB ,QB > IA,QA

QC E1C - VGRID B

IC ,QC > IB ,QB > IA,QA

E1 C

iexc

Representative straight lines of the voltage drop on the

reactances Xd1 (yellow part) and Xe1 (white part)

QA E1A - VGRID A

QB E1A - VGRID B

IB ,QB > IA,QA

QA

E1A

- VGRID A

QB E1A - VGRID B

IB ,QB > IA,QAV GRID A

Internal gen.voltage E1

Terminals gen.voltage V1 V GRID

V1AE1 A

xe1xd1

Inside the

Generator V1B V GRID B

Drop voltage on the

grid with consequent

drop voltage on V1( from V1A to V1B )

1

2

Behaviour on a near short circuit


During a strong and near short circuit on the grid, the voltage at generator terminals strongly lowers while,

on the contrary, the statoric current increases correspondingly.


111/141

Doc. n 196 W 429 Rev.1 Oct. 2008 110

Forcing of the automatic

regulator in the attempt to

sustain the voltage at the

terminals of the generator

Internal voltage ofgeneratorE1

V GRID

V1AE1 A

xe1

xd1

Inside the Gen.

V1B

VGRID B

QC E1C - VGRID B

IC

,QC

> IB

,QB

> IA,Q

A

QC

E1C

- VGRID B

IC

,QC

> IB

,QB

> IA,Q

AE1 C

iexc

Voltage drop on

the grid due to the

short cicuit

Voltage at gener.terminals V1

on the contrary, the statoric current increases correspondingly.

The electromagnetic flux in machine [ total = rotor - stator] strongly decreases a lot immediately,

therefore the magnetic connection between rotor and stator becomes very weak and so the mechanical couple

of the turbine, still strong because slower to be reduced, can cause the tear of the weak magneticconnection and it can produce the so-called out of step.

This is avoided, at times, if the short circuit duration is so brief that the excitation current, quickly increased

due to the ceiling, has had time to create a flux in machine such to be still able to withhold rotor and stator

connected among them.

V GRID ACorrection on the

terminals voltage due

to the ceiling

Electric grid systemEach generator synchronised to the grid contributes to supply the grid load by its

own stator current (I1,, In) ----> Itot = i IiIncreasing the generators number, both the total current Itot and the global grid



112/141

Doc. n 196 W 429 Rev.1 Oct. 2008 111

E

G 1

xd1 xe1ie1

E1 V1

E

G 2xd2 xe2ie2

E2 V2

E

G n

xdn xenien

En Vn

I1

I2

In

Grid loads

Z tot = very low

n generatorssynchronised on

the grid

I tot

V GRID

E

G AxdAxeA ieA

EAVAIA

g g , tot g g

power increase consequently P = 3 * VGRID * I tot .The total grid impedence Ztot can be difined as ratio between the grid voltage

VGRID divided by the total grid current Itot in order to satisfy the relation

VGRID = Z tot * I tot .

When an other new generator is connected to the grid, its contribution IA to feed

the grid loads is added to the preexistent Itot

and the grid voltage will be modified

by that contribution with the same rate by which IA modifies Itot --> IA / Itot gives

V on the grid.VGRID = Z tot * (Itot + IA) if IA


113/141

Doc. n 196 W 429 Rev.1 Oct. 2008 112

E

G 1

xd1 xe1ie1

E1 V1

E

G 2xd2 xe2ie2

E2 V2

E

G n

xdn xenien

En Vn

I1

I2

In

Grid loads

Z tot = very low

n generatorssynchronised on

the grid

I tot

V GRID

E

G AxdAxeA ieA

EAVAIA

G 1G 2G 3G nG A

Grid

loadsabsorption

The tank level is mainly due and maintained by

the big set of sources G1Gn balanced by the

grid loads absorption.

The new source GA is influent on the tank level as

far as its contribution is relevant compared with

the global contribution of all other sources.

The electro-hydraulic comparison is significant inorder to emphatise the influence of a single generatorcontribution on the grid voltage alteration, when it isconnected to a powerful national grid.

The regulation system provided into the exciter is normallycomposed by two identical digital automatic voltage



114/141

Doc. n 196 W 429 Rev.1 Oct. 2008 113

composed by two identical digital automatic voltage

regulator (AVR) operating in alternative (master & slave).

At starting, each AVR can be selected as master.Being the stand-by regulator automatic type too (not manual)

all functions of the main one are again available for the

best dynamic control of the generator behaviour.

The generator voltage feedback is doubled (one each AVR)

and the feeders too in order to have 100% redundancy.

A logic system (pls. see the following image) oversees the

good operation of the AVRs and, in case of problems ofAVR master, decides to switch-over to the second AVR.

Regulation and Control



115/141

Doc. n 196 W 429 Rev.1 Oct. 2008 114

A V R

Automatic regulator

AVR 1

Stator voltage and currentfeed-back (for channel 1)

V stator

I stator

A V R

Automatic regulator

AVR 2

Feeding

V stator

I stator

Control/Protection Logic

I/O to

remote

Converter 1

Converter 2

Plant

Stator voltage and current

feed-back (for channel 2)

Feedbacks of the regulators



116/141

Doc. n 196 W 429 Rev.1 Oct. 2008 115

VT1

VT2

CT-U

CT-W

For the

regulator1

For theregulator2

For regulators

1 e 2

Generator

Vc

tensione

statorica

concatenata

Set of 3 VTssingle phase for

the voltage

feedback of the

machine

Set of 3 VTs

single phase for

the voltagefeedback of the

machine

CTs for the

current feedback

of the machine

Main breaker of

the machineEvery regulator works correctly if only

it is constantly informed on the

actual values of voltage and currentproduced on the stator.

For this purpose there are VTs and CTsconnected as in the figure.

Every set of 3 VTs feeds its own

regulator making in this way twoloops of stator voltage regulation

completely independent.

The CTs feed in series both the

regulators and they are used to

perform auxiliary functions(compound, limits, etc.).

The following image shows a list of the main functions

normally foreseen into each AVR (with optional too)



117/141

Doc. n 196 W 429 Rev.1 Oct. 2008 116

normally foreseen into each AVR (with optional too).

Some note on them.

- Each of the two AVRs has the possibility to operate, locally,in manual mode too (commissioning phase) through the

display and the keyboard mounted on the cubicle front.

- The function called Power System Stabilizer (PSS) is usedto strongly reduce the oscillations (amplitude and time) on

the stator machine variables, due to sudden load changes.

- The best tuning of the PSS parameters have to be found by a

complex plant/grid study, normally not given by theexciter supplier, but produced by the plant designer only.

Typical functions of the automatic voltage regulation



118/141

Doc. n 196 W 429 Rev.1 Oct. 2008 117

Automatic stator voltage regulation

Manual field voltage regulation

Reactive current compensation (pos. and neg.)

Stabilizing signals function (PSS)

Automatic reduction to zero of reactive power

Automatic cos regulation

Automatic reactive power regulation

Voltage calibrator Max field current limit

Underexcitation limit

Voltage/frequency limit

Automatic alignment with the other AVR

Field winding temperature

Max stator current limit

Operating functions

of the AVR regulator

The green functions are optional and

applicable in certain cases only.

SFC interface and logic

AUTOMATIC VOLTAGE REGULATOR


F db k f t t


119/141

Doc. n 196 W 429 Rev.1 Oct. 2008 118

+ -Ref.

Feedback of stator

voltage

Error

+

-

Ref.

Feedback

of stator

voltage

K11 + s T1

s

+

-

K31 + s T3

s

Feedback of

field voltage

Thyristors

firing angleA B

Regulator Firingcircuits

Regulation systemOperator

interfaceG

Feedback of stator

voltage

Bridge

Transfer functions

AUTOMATIC VOLTAGE REGULATOR


The automatic regulation has the main purpose to maintain constant the voltage at

the alternator terminals even if the load varies casually.


120/141

Doc. n 196 W 429 Rev.1 Oct. 2008 119

Vstator

time

V0 V0

transitory

0

Reference

Error negative

( reduce Iexc)

Errors positive

(increase Iexc)

Feedback

time

Error Error = Reference - Feedback

Error = 0

0

The figure at left shows, for instance, the behaviour of the stator voltage of a

machine that sees a load disconnection and reacts with its regulation.

The voltage increases but the regulator reacts and brings it back to the previousvalue V0 after some oscillation (for simplicity in this example the contribution of

the function compound is not considered pls. see next image).

When the feedback changes the error on the reference corrects the Iexc.

COMPOUND AND COMPENSATION


This function automatically corrects the set-point of the regulation loop of the stator

voltage in proportion to the actual current reactive delivered by the generator Such


121/141

Doc. n 196 W 429 Rev.1 Oct. 2008 120

voltage, in proportion to the actual current reactive delivered by the generator. Such

correction can be in increases (straight line in rise- compensation) or in decreases

(straight line in descent-compound) but it is always done in automatic and transparent

way for the operator.

It is an additional signal injected in the comparison point that corrects the reference of the

regulator.

The compensation allows to maintain constant the voltage after the step-up transformer.

The compound contributes to the automatic reactive load division among more groups

of the same plant in parallel on the same bar (v. figures in the following slide).

+-

Ref.

Feedback ofstator voltage

+

Compound Other

signals

1 p.u. di Ireact =

I nom statoric

Ireactiva

Vstator

1 p.u. Band

0 22%

Band

0 22%

Compensation

Compound

0

Error

DIVISION OF LOAD


High voltage V0

The load connection produces arapid lowering of the high voltage

V0 that goes from here (V01) to

here (V02)IB1IA1

V1 V2

V

Generator 1 Generator 2


122/141

Doc. n 196 W 429 Rev.1 Oct. 2008 121

In this case both the reactive currents IA1 and IB1 increase up

to IA2 and IB2, but in different rate if the inclinations of the

segments orange and blue (the compound) are different.

In this case the load is not divided equally.Ireact

Ireact

The load connection produces a rapid loweringof the high voltage V0 that goes from here

(V01) to here (V02)

IB1

IB2

IA1

IA2

V1 V2

V01

V02

Ireact1Ireact2

G1 G2

T1 T2

V1 (M.V.) V2 (M.V.)

IreactIreact

In the orange diagrams the Ireact increases to the

right from left, while in those blues the Ireact

increases to the left from right

Ireact

Ireact

B1

IB2

A1

IA2

The reactive currents IA1 and IB1 increase equally up to IA2 and IB2 if the

inclinations of the segments orange and blue (the compound) are the same.

In this way the load is equally divided.

V01

V02

Ireact1 Ireact2

STABILIZING SIGNALS ( PSS )


Purpose of this function is to reduce the electromechanical oscillations of the machine

due to consistent load variations and this favors the stability of the group and of the grid


123/141

Doc. n 196 W 429 Rev.1 Oct. 2008 122

Apex of theoscillation

Pendulum

to it connected.

Its intervention is transitory (just for the strong load variations only) and the principle of

its operation is to add brief corrective impulses to the field current in the mostopportune instants of the oscillations: in such way the damping action will be favorite.

An intuitive comparison that can clarify the mechanism concept of

this function operation is illustrated in figure. In the moments near

to those in which the pendulum reaches the apex of its oscillation

it is possible to influence its future run giving new touches of

push, also of modest entity, but in well precise instants. If such

touches of push are applied, every time, immediately after the

pendulum has already reached and overcome the apex of every

run, the ampleness of its oscillations will increase more always.

Contrarily, if that touches of push are applied, every time,

immediately before the pendulum has reached the apex of everyrun, the ampleness of its oscillations will find contrast and it will

decreaseprogressively.

This is the same principle of action of the stabilizing signals.

STABILIZING SIGNALS ( PSS )


The two following figures show what is the effectiveness of the

bili i i l h d h i ill i


124/141

Doc. n 196 W 429 Rev.1 Oct. 2008 123

-1 0 1 2 3 4 5 6 7 80

0.5

1

1.5

P

-1 0 1 2 3 4 5 6 7 80

0.5

1

1.5

P

Without PSS With PSS

stabilizing signals on the process to reduce the statoric oscillations

produced by a strong transitory of power on the grid (for instance

short circuit).

The setting of the PSS parameters for a generator connected to a particular electric grid can

normally be optimized with specific SW simulations, given by the Manager of the Grid.

REGULATION OF THE REACTIVE POWER


The regulation of the reactive power Q is done with the help of the regulation loop of the statoric voltage,

through which, regulating the field current (and therefore the internal voltage Ei), it is possible vary the


125/141

Doc. n 196 W 429 Rev.1 Oct. 2008 124

Internal voltage E1

Grid voltage VGRID

(E1 VGRID ) Q

Time

Voltages

0

Correcting E1 in order to recopy VGRID, the Q will be maintained constant

through which, regulating the field current (and therefore the internal voltage Ei), it is possible vary the

reactive power that the generator exchanges with the load (as already seen previously).

With this type of regulation, Q is maintained constant maintaining constant the difference between the

internal voltage Ei and the grid voltage VGRID.Practically there is a regulation loop in which the wished reference Q* is continuously compared with the

real reactive power currently delivered: the possible error corrects the reference of the voltage loop to vary

Ei in such way that the generator will produce a Q (= Ei-VGRID) equal to the wished reference Q*.

REGULATION OF THE POWER FACTOR


The regulation of the power factor (cos ) is done with the help of the regulation loop


126/141

Doc. n 196 W 429 Rev.1 Oct. 2008 125

MW

MVAROver-excitation

Under-excitation

MW2

MVAR2

MVAR1

MW1

P1

P2

P1

0

g p ( ) p g pof statoric voltage, through which it is possible vary the reactive power exchanged by

the generator with the load (as already seen previously) and consequently its cos .

The working principle of this type of regulation is

the following.

Continuously it is measured the active power

value that the machine is delivering now and,

based on it, it is calculated the consequent

reactive power that maintains constant thewanted relationship among these two powers

(active and reactive) and therefore the cos .

The automatic regulator will act to maintain this

calculated reactive power until there will be a

successive variation of the active power.

The figure shows that, if the MW changes (from

MW1 to MW2), the exciter will vary the MVAR

(from MVAR1 to MVAR2) and P1 P2.

LIMITS IN OVER AND UNDEREXCITATION ZONES


MW Action curve of the

overexcitation limit


127/141

Doc. n 196 W 429 Rev.1 Oct. 2008 126

LIMIT IN OVEREXCITATION

This function protects the generator against

prolonged trespasses out of the capabilityzone hazardous for the integrity of the field

windings.

LIMIT IN UNDEREXCITATION

This function protects the generator against

prolonged trespasses out of the capability

zone hazardous for the stability of the group

and for the integrity of terminal parts of the

stator.

MVAR

0

Overexc.Underexc.

+

overexcitation limit

MVAR

Nominal workingpoint Pn

MW

0

Overexc.Undrexc.

_

Action curve of the

underexcitation limit

LIMIT OF FLUX


Th fl i id h hi i i b h i V/f


128/141

Doc. n 196 W 429 Rev.1 Oct. 2008 127

f (%)

V (%)

100%95%

95%

105%

105%

98%

103%

102%

Flux V/f == 105% nom

Flux V/f == 95% nom

The flux inside the machine is given by the ratio V/f .

The colored area represents the surface of the normal

working points allowed indefinitely by the project,while the outlined areas adjacent to it, are always

representative of working points allowed for the

machine, but with more stressful conditions of those

in the colored zone.

For this reason the permanent exercise of the group

outside the colored area is unadvisable, in order to

limit, as far as possible, the "loss of life" of the

generator.

The flux limit of the exciter acts along the highest

oblique segment and it adapts the stator voltage on the

foreseen relationship V/f, when the frequencydecreases.

.


V (%)

In this diagram V/f the limitations are

due to:


129/141

Doc. n 196 W 429 Rev.1 Oct. 2008 128

A

D

B

C

f (%)

100%95%

95%

105%

105%

98% 103%102%

Flux V/f == 105% nom

Flux V/f == 95% nom

in A

max flux (V/f) , max voltage (gen.)

and min frequency (turbine)

in B

max voltage and max frequency

in C

max frequency, min voltage (max

current) and min flux (instability)

in D

min frequency and min voltage

(max current)

The reactive power and power factor regulations can be only


NOTES


130/141

Doc. n 196 W 429 Rev.1 Oct. 2008 129

- The reactive power and power factor regulations can be only

actuated when the generator is synchronized to the grid.

Its disconnection automatically produces a change-over to

the stator voltage regulation (in island configuration too).

- Any change-over to the other regulator is bumpless in

order to avoid any dynamic and thermal machine variation.

Typical exercise sequence of a generator



131/141

Doc. n 196 W 429 Rev.1 Oct. 2008 130

Predisposition completed

Turbine at nominal speed

Exciter fed and in automatic

voltage regulation

Exc

ON

Exc

OFF

Soft-start

Voltageequalization

First loading

(few MW)

INCR / DECR

EHC and EXCLOADING

Reduction of

Reactive

Power to 0

Reduction of

Active

Power to 0

Disconnection

from the grid

LOADINGVOLTAGE

Regulation

Synchronization

Reg. COS orReg. REACT.

Generator

de-excited

FRONT VIEW OF A TYPICAL EXCITATION BOARD



132/141

Doc. n 196 W 429 Rev.1 Oct. 2008 131


INTERNAL VIEW OF TYPICAL POWER CUBICLES


133/141

Doc. n 196 W 429 Rev.1 Oct. 2008 132


TYPICAL MODULE WITH THYRISTORS AND RADIATORS


134/141

Doc. n 196 W 429 Rev.1 Oct. 2008 133



135/141

Doc. n 196 W 429 Rev.1 Oct. 2008 134

EXCITATION

TRANSFORMERS

EXCITATION TRANSFORMERS


The feeding transformers of the excitation systems are substantially


136/141

Doc.

Documents

9-Generator Performance Curves and Static Exciation