Automatic Voltage Regulator for Generator

7/28/2019 Automatic Voltage Regulator for Generator

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

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 to

have 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.

AnsaldoEnergia Una Societ Finmeccanica


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Automatic and Manual regulation modes

Automatic regulation

mode

GENRegulation& Excitation

systems

Stator

voltageElectric

feed-back

Set-point

selection

Automatic

correctionManual regulationmode

GEN

Stator voltage

indicator

Human visualfeed-back

Continuous human

correction

Regulation& Excitation

systems



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CLASSIC REGULATION LOOP

OF THE STATOR VOLTAGE


GiringcircuitsegulatorErr.

Feedback

Reference

VT feedback

Bridge

+

-

Exciter


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AUTOMATIC VOLTAGE REGULATOR


+-

Ref.

Feedback of stator

voltage

Error

+

-

Ref.

Feedbackof stator

voltage

K11 + s T1

s

+

-

K31 + s T3

s

Feedback offield voltage

Thyristors

firing angleA B

Regulator Firingcircuits

Regulation systemOperatorinterface

G

Feedback of stator

voltage

Bridge

Transfer functions


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AUTOMATIC VOLTAGE REGULATOR


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

the alternator terminals even if the load varies casually.

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 previous

value 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.


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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

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 transparentway 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.

+ -

Ref.

Feedback of

stator voltage

+

Compound Other

signals

1 p.u. di Ireact =

I nom statoric

Ireactiv

a

Vstator

1 p.u. Band

0 22%

Band

0 22%

Compensation

Compound

0

Error


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STABILIZING SIGNALS ( PSS )


Apex of the

oscillation

Pendulum

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

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 most

opportune 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 nearto 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 every

run, the ampleness of its oscillations will find contrast and it will

decreaseprogressively.

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


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STABILIZING SIGNALS ( PSS )


-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

The two following figures show what is the effectiveness of the

stabilizing signals on the process to reduce the statoric oscillations

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


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REGULATION OF THE REACTIVE POWER


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

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

reactive power that the generator exchanges with the load.

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*.


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REGULATION OF THE POWER FACTOR


MW

MVAROver-excitation

Under-excitation

MW2

MVAR2

MVAR1

MW1

P1

P2

P1

0

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

of 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 the

wanted 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 asuccessive 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.


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LIMITS IN OVER AND UNDEREXCITATION ZONES


LIMIT IN OVEREXCITATION

This function protects the generator against

prolonged trespasses out of the capability

zone hazardous for the integrity of the field

windings.

LIMIT IN UNDEREXCITATION

This function protects the generator against

prolonged trespasses out of the capabilityzone hazardous for the stability of the group

and for the integrity of terminal parts of the

stator.

MVAR

MW

0

Overexc.Underexc.

+

Action curve of the

overexcitation limit

MVAR

Nominal working

point Pn

MW

0

Overexc.Undrexc.

_

Action curve of the

underexcitation limit


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LIMIT OF FLUX


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 highestoblique segment and it adapts the stator voltage on the

foreseen relationship V/f, when the frequency

decreases.


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

.


A

D

B

C

f (%)

V (%)

100%95%

95%

105%

105%

98% 103%102%

Flux V/f =

= 105% nom

Flux V/f =

= 95% nom

In this diagram V/f the limitations are

due to:

in Amax 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)


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- The reactive power and power factor regulations canbe only actuated when the generator is synchronizedto the grid.

Its disconnection automatically produces a change-

over to the stator voltage regulation (in islandconfiguration too).

- Any change-over to the other regulator is bumplessin order to avoid any dynamic and thermal machine

variation.


NOTES


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

Typical exercise sequence of agenerator


Predisposition completed

Turbine at nominal speed

Exciter fed and in automatic

voltage regulation

Exc

ON

Exc

OFF

Soft-start

Voltage

equalization

First loading

(few MW)

INCR / DECR

EHC and EXCLOADING

Reduction of

ReactivePower to 0

Reduction of

ActivePower to 0

Disconnection

from the grid

LOADINGVOLTAGE

Regulation

Synchronization

Reg. COS orReg. REACT.

Generator

de-excited

Automatic Voltage Regulator for Generator

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