avr on load tap changer

8/10/2019 AVR on Load Tap Changer

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On/Under Load Tap Changing Transformer

Automatic Voltage Regulating Control Relay


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On/Under Load Tap Changing Transformers Voltage Regulating Control Relays3 3

Contents

On Load Tap Changing Transformers

Voltage Regulation

Time Delays

Grading Between Voltage Levels

Line Drop Compensation

Parallel Operation


4/39On/Under Load Tap Changing Transformers Voltage Regulating Control Relays4 4

Voltage RegulationPurpose

Electrical plant is designed to operate within finite

voltage limitsTo control the system voltage transformers with tapchangers are commonly used

Off Load manually changedOn Load automatically adjusted

Tap changers physically alter the transformer ratio and

hence the voltage Automatic voltage regulating relay is used to control thetap changer



Voltage RegulationPerformance Criteria

Two Mutually Exclusive Parameters

voltage quality

number of tap changes

Voltage Quality

customer focused quality measurement

percentage of time outside deadband or statuary limits

length of voltage disturbance

Number of Tap Changes

supplier focused quality measurement

directly relates to cost of ownership of the tap changer



On Load Tap Changing Transformers



On Load Tap Changing TransformersTypical Supply Network



On Load Tap Changing TransformersTap-Outs and Windings

Tap-outs can be in the HV or LV winding

Tap-outs can be at the Line End or Neutral End

Cause a variation in the impedance of thetransformer

Transformer impedance calculated at thecentre tap position



On Load Tap Changing TransformersDevelopment of the Tap Changer

Developed in the late 1920s

Two primary considerations

The load current must not be interrupted

No windings should be short-circuited

during transitionsEarly tap changers were reactors, could becontinuously rated useful when mechanisms

were unreliableResistor transition tap changers now used,reliable stored energy drive mechanisms

Typical Operation ~75ms



Voltage Regulation


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Voltage RegulationExample Operation

+ VoltageDeadband

- VoltageDeadband

VoltageSetting

Time Delay

t

V

Voltage outof deadband

Voltage restoredto within deadband


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Time Delays


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Time DelaysInitial Time Delays

Initial time delay used to filter transient voltage fluctuations

definite time

inversely proport ional to the deviation from the voltage setting

Operating Time

VoltageV B an

d +

V B an d -

V S e t t i n g

Definite TimeCharacteristic

Inverse Time

Characteristic

l i l Ch S


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Multiple Tap Change SequenceInter-Tap Delay - Definite Time

VoltageDeviation

Vs+dVs

Initial DelayInitial Delay Definite Time)

Time

M l i l T Ch S


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Multiple Tap Change SequenceInter-Tap Delay - Inverse Time

VoltageDeviation

VsdVs

Initial Delay Inverse Time)

Time

The timer is reset after each operation to invoke init ial timer

I i ti l Ti D l


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Initial Time DelaysTypical Setting Ranges

Initial Time Delay Setting Ranges

MVGC - 0 to 120s

KVGC - 0 to 300s

Inter-tap Delay Setting Ranges

MVGC - 0 to 10s

KVGC - 0 to 120s

Initial Time Delays also ensure time grading betweenvoltage levels


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Grading Between Voltage LevelsCauses of Voltage Deviations

Grid Supply

Load

Load

A

CB

Line

T3T2

T1

Load

Voltage deviations occur for two reasonsChange in downstream load

Change in upstream supply voltage



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Grading Between Voltage LevelsTime Grading

Grid Supply

Load

Load

A

C

BT3

T2

T1Load

Discrimination to allow higher voltage to operate firstT1 time delay < T2 time delay < T3 time delay (initial)

Otherwise hunting can occur between voltage levels


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Line Drop CompensationRegulating the Voltage at a Remote Point

Tap Changer

Time Delay

VoltageDeadband

Voltage Setting AVC Relay

Supply

Load

VT

CT

+-

Bus

X R

Feeder

Voltage Drop

Line Drop Compensation )(

FEEDERFEEDER LOADS LOAD jX R I V V +=

)( AVC AVC AVC S BUS jX R I V V +=

Simulates voltage drop of the line

Artificially boosts transformer voltage at times of high loading



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Line Drop CompensationRelay Implementation

Calculate the Voltage Drop at nominal load

Vr Vxl

RemoteVoltage

VREM

LoadIL

BusVoltage

VB

Bus Voltage VBUS

= VREM

+ Vr + Vxl



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Line Drop CompensationPhasor Diagram

IL

VBUS

-ILX

-ILR

VREM Voltage Setting

The relay calculates andregulates the V REMmagnitude

The measuredbus voltage



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Line Drop CompensationTypical Setting Ranges

Reactive and Resistive Compensation, Vr andVxl

MVGC - 0 to 48V

KVGC - 0 to 50V


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Transformers

Parallel Operation

Parallel Operation


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pTransformer Operating in Parallel

Transformers in parallel can cause CirculatingCurrents

Circulating Current affects LDC causing tapchanger runaway

Circulating Current are damaging to Transformer

Therefore special control techniques are required

Paralleling using Master-Follower Control

Paralleling using Negative Reactance Control

Paralleling using Circulating Current Control

Parallel Operation


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pMaster - Follower

One controller designated the master

All other controllers (followers) follow the master

Requires that

Identical transformers / tap changers

Locality of equipment

Security of communications is crit ical

Circulating current should be monitored

Parallel Operation


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pCirculating Currents due to Tap Disparity

2IL

T1

T2

Ic

IL-Ic

IL+Ic

If T1 is on a highertap than T2

Current seen by#1 is I L + IC

Current seen by#2 is I L - IC

#2

#1

Parallel Operation


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pCirculating Current Effect on LDC

2IL

T1

T2

Ic

Vxl VrIL-Ic

IL+Ic

IC is measured in LDC circuit but

is not present in the feeder

#2

#1

Parallel Operation


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LDC on T1

IL

VBUS

-ILX-ILR

VREM

ICIL + I C Volts High)

VREG

-IcR

-IcX

The regulated voltage V REGcontains a -V C component

VREG will be regulated up to V REM ,therefore increasing the overvoltage even further

Voltage Setting

The regulatedvoltage



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Parallel Operation


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Negative Reactance Control

Reverse Reactance Control (non matched tap

changers or different sources)Utilises the reactance compensation of the LineDrop Compensation circuit

Reverses reactance and feeds back intocompensation circuit

Lack of specific feeder reactance in LDC resultsin susceptibility to power factor



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Compensation on T1

IL

VBUS

VREM

ICIL + I C Volts High)

VREG

Voltage Setting

ILXt-ILR-ICRC

Xt

The measured

bus voltage


VREM

VREG

-ILX

-ILRIcR

-IcX

The reactance of LDC is reversed

VREG will be regulated correctlyreducing the over voltage



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Compensation on T2

IL

VBUS

-ILX

-ILR

VREM

IC

VREG

-ICR

-ICX

Voltage Setting

ILXt

-ILR-ICR

ICXt

The reactance of LDC is reversed

VREG will be regulated correctlyincreasing the under voltageL - IC Volts Low)



VREM

VREG


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Supplementary Control Functions AVR Relaying

Supplementary Control Functions


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Load Sheddingvoltage setting override ( 10%)

Over Current / Under Voltage Supervision

inhibit control during fault conditions

Excessive Circulating Current

protects against parallel control failure

Under Current Inhibit

stops operations when low forward or reverse power

Reverse Current Operation Auto/Manual/Remote Operations

Measurements

Supplementary Control FunctionsT Ch S i i


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Tap Changer Supervision

Tap Changer Monitoringtap changer confirmation response (time limited)

voltage monitoring

voltage change occurs

in the correct direction

of sufficient magnitude

operations with no initiating signalTap Change Operations Counter

alarms at settable threshold

frequency alarmsexcessive operations per time period(day / month / maintenance period)


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avr on load tap changer

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