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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.
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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
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GiringcircuitsegulatorErr.
Feedback
Reference
VT feedback
Bridge
+
-
Exciter
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AUTOMATIC VOLTAGE REGULATOR
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+-
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
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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
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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 )
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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 )
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-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
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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
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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
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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
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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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.
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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.
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NOTES
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Typical exercise sequence of agenerator
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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