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auto volt regulate

Jun 02, 2018

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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