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AVRAutomaticVoltage
Regulator
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EXCITATION SYSTEM
AVR-1
AVR-2
AVR
Manual
Pulse
Generator
AVR
Manual
Pulse
Generator
Thy.
Bridge-1
Thy.
Bridge-2
~
GB
Bus
Gen
Rotor
Field
Flashing
Rectifier
Field
Breaker
Ex. Tr.
AC
DC
AC
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Causes for voltage variation
The variation in terminal voltage of the
machine, on load condition are due to the
following causes:
Voltage variation due to the resistance ofthe winding R
Voltage variation due to the leakage
reactance of the winding XlVoltage variation due to the armature
reaction effect Xa
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Types of Voltage Regulation
Manual Voltage Regulator
open loop control system without
feedback.
Automatic Voltage Regulator
closed loop control system with
voltage feedback.
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FUNCTIONS OF AVR
To maintain constant generator terminalvoltage.
Meet excitation power requirements under allnormal operating conditions.
Improve dynamic and transient stabilitythereby increasing plant/machine availability.
Regulate MVAR loading within limits.
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dvantages of VR
1. The response time for anyvariation is less.
2. Steady state error is small.
3. The settling time is less.
4. The system remains stable for awide range of operating points.
5. Limiters can be incorporated.
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Types of AVR
1. Analog type AVR
2. Digital type AVR
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Advantagesof Digital AVR
Lesser number of Electronic cards.
Better configuration and control.
Fast reaction to network disturbances.
Troubleshooting easier.
User- friendly software can be used forsetting parameters and measuring variables.
Easy adaptation to customers requirements
( logic changes ) even at site without anyhardware modifications.
Self monitoring and fault diagnosis features.
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Automat ic Voltage Regu lator
The Control system keeps the Generatoroutput voltage constant by controlling the field
energy supplied to the rotor.
Control
Voltage
ErrorSignal
Increase
Decrease
Comparator
Gen. Voltage
FeedbackLimiters
Reference
Value
Generator
Actual
Value
Generator
+
-
PID
Controller
Firing Pulse
Generator
IExe VExe
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Voltage Feedback measurement
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Excitation system
performance could be
judged by the exciter voltage
VStime characteristics inresponse to a step change in
the generated voltage.
The factors to be studied for
optimum performance are1. Overshoot
2. Rise time
3. Settling time
For ideal performance, it should have one overshoot and one
undershoot with a quicker rise time.
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PID CONTROLLER
The objective of any closed loop control system isto keep the following parameters in check
The overshoot after any step change in controlledvariable (in this case the voltage) should be within
acceptable limits for the system to be stable.
The response time should be as small as possible so
that the corrective action to any change in the
controlled variable is taken by the controller without
any appreciable lapse of time.
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Steady state error should be as small as possible.
The settling time should be as small as possible.
The system should remain stable for a widerange of operating points.
To improve the performance, compensationnetworks are provided in the control amplifier.
These compensation networks are namely
Proportional, Integral and Derivative feed back
circuits.
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PID CONTROLLER
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Features of VR
Feature Purpose
Reactive Current
Compensation
To compensate the external reactance of the group
transformer or to share the reactive load among
different parallel generators.
LimitersTo maintain field current and stator current and to
operate the machine within capability limit.
Power System
Stabilizer
To smoothen the electro mechanical oscillations.
Reactive Power
Control
To regulate the reactive power if necessary.
Power Factor
Control
To regulate the power factor if necessary.
Rotor Temperature
Calculation
To calculate the rotor winding temperature from
Rotor current and rotor voltage.
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CAPABILITY DIAGRAM OF GENERATOR
In power system operation, the importance and
necessity of fast acting and reliable excitation controlsystem is well known.
Capability diagram of Generators give the safe
operating regimes and limitations etc. This is of great
help to the operating Engineers to ensure operations of
the machines accordingly.
Their information particularly for limiting zones of
operations are useful in setting the various limiters ofAutomatic Voltage Regulator.
MW values are marked on Y axis and MVAR values
on X-axis on per unit basis of rated MVA.
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CAPABILITY DIAGRAM OF GENERATOR
safety factor a 12.5 percent (or 1.125 p.u) powermargin to increase in power demand with nocorresponding increase in excitation PracticalStability Limit Line.
(125 x 0.6)=75 MVAR i.e. 0.6 pu From the point Athe dotted line ASdenotes the
theoretical stability line.
Horizontal lines parallel to X-axis denote the MW(constant power) lines.
The diagram FBED is the CapabilityDiagram ofthe machine.
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LIMIT CONTROLLERS
With ever increasing size of generating units
today, more stringent requirements have to be met byexcitation systems.
Static Excitation assures, stable operation both
under dynamic and transient conditions. Generators running in parallel with the power
network even under extreme conditions must remain
in synchronism with the maximum load limit on it
being not exceeded. Optimum utilization of the generator can be
ensured only if the basic AVR is influenced by
additional limiting signals.
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ADVANTAGES OF LIMIT CONTROLLERS
Limit controllers in conjunction with AVR ensure
optimum utilization of the machine. Limit controllers simplify the job of the operating-
staff.
Limit Controllers enable stable operation of themachine close to the limiting values.
With limit controllers in service, operational errorsand faults in the regulator lead only to the limit
value control and not to disconnection.
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ADVANTAGES OF LIMIT CONTROLLERS
Limit controllers are not meant to replace theprotection system.
They are intended to prevent the protectionsystem from operating under extreme transient
conditions.
Limiters, whenever they intervene, influence thevoltage regulator suitable to bring about a
corresponding change in the excitation.
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Rotor current limiter
The AVR drives the field or the Thyristor network into
overload for one or more of the following reasons :
a) faulty handling
b) system voltage reduction
c) loss of sensing voltage to AVR andd) failure within the controller.
The Rotor current limiter must prevent this overload frompersisting.
On the other hand, during dynamic disturbances in thesystem the excitation should not be reduced at once, butceiling excitation should be possible for a limited time.
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ROTOR CURRENT LIMITER
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FUNCTION OF ROTOR CURRENT LIMITER
The field current is measured on thea.c. input side of the thyristor converter
It is converted into proportional d.c.
voltages.
The signal is compared with an
adjustable reference value, amplified,
and with necessary time lapse fed to thevoltage regulator input.
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ROTOR CURRENT LIMITER
Rotor current limiter avoids thermal overloading of
the rotor winding and is provided to protect thegenerator rotor against excessively long duration
over loads.
The ceiling excitation is limited to a predetermined
limit and is allowed to flow for a time which is
dependant upon the rate of rise of field current
before being limited to the thermal limit value.
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ROTOR CURRENT LIMITER
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ROTOR CURRENT LIMITER
Simple mode:In this mode the excitation current is
limited to a preset maximum value. The limiterintervenes with a time delay which is proportional to themagnitude of the over load.
Mixed mode:If during the above period of limitation,
the generator voltage dips steeply for any reason, theceiling excitation limit is validated again. The ceilingexcitation current helps in increasing the short circuitcurrent in the fault zone and hence aid selective trippingof the faulted section.
Switching mode:In the switching mode the excitationis limited to the thermal or rated current value. Only incase of sharp dip in the machine voltage, the ceiling limitwas unable momentarily. The limit switches back to therated value after the set time.
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Stator current limiter
The stator current limiter has to influence the
AVR differently depending on whether themachine is over-excited or under-excited.
The excitation current is to be suitablyreduced to limit the inductive stator current.
The excitation current is increased to limit thecapacitive current.
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Stator current limiter
Capacitive stator current limitation comes intoplay only with synchronous condensers whichare to some extent negatively excited withgenerators.
It prevents excessive leading MVAR loadingcorresponding to any given MW load.
STATOR CURRENT LIMITER
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STATOR CURRENT LIMITER
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Stator current limiter
The generator stator current is converted intopolarised dc signal +ve orve, depending uponwhether the machine is over-excited or under-excited.
This voltage forms the actual value for thecontrollers which process each of the bipolar signalindependently.
One of the these controllers compare thecapacitive stator current against its reference andacts directly on the regulator via a de-coupling
diode to increase the excitation.
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Stator current limiter
The action of second controller which limits theinductive stator current is delayed by means of anintegrator before it influences the control input ofthe AVR so as to reduce the excitation.
The time lag offered is perfectly acceptable as faras stator overheating is concerned.
The integrator time constant is set one order less
than the stator thermal time constant.
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Rotor Angle Limiter
The load angle is the electrical angle between the
voltage vector of the system and the vector of themachine voltage e.
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Rotor Angle Limiter
In the event of a short circuit in the systems, the
generators may accelerate owing to the abrupt partialremoval of the electrical load.
As the turbine governor cannot act fast, the rotor angle
increases and the angle can become so large relative tothe system vector that the machine may fall out of step.
The rotor angle limiter limits the load angle of the
machine to an acceptable present value and provides amore definite protection in preventing the machine fromfalling out of step.
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ROTOR ANGLE LIMITER
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Minimum Excitation Limiter
Minimum Excitation limiter measures the reactive
power and compare it with the set reactive power
in the leading side . It gives positive output toincrease the voltage regulator reference.
This Limiter assures to feed the generator the
minimum excitation that is necessary to keep the
synchronization defined by the capability curve.
Power System Stabilizing unit
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Power System Stabilizing unit
In large power system network sudden change in
load sets up all kinds of oscillatory responses. These
oscillations are essentially exchange of power orenergy between rotors of machines.
The Power system stabilizing unit is used for the
suppression of rotor oscillations of the machines
through the additional influence of excitation.
The power as well as acceleration signals needed for
the stabilization are derived from active power
delivered by the alternator.
Both the signals, which are correspondingly amplified
and summed up, influence the excitation of the
synchronous machine through AVR in a manner as to
suppress the Rotor oscillations.
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AutomaticVoltage Regulator
Maintaining terminal voltage of a Generatorconstant.
Maintaining stable operation of machine under
steady state, transient and dynamic conditions.
Satisfactory operation with other machines
connected in parallel.
Effective utilization of machine capabilitieswithout exceeding machine operating limits.
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THANK YOU