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8/10/2019 AVR on Load Tap Changer
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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
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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
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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
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On Load Tap Changing Transformers
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On Load Tap Changing TransformersTypical Supply Network
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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
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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
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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 Levels
Grading Between Voltage 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
Grading Between Voltage Levels
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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 Compensation
Line Drop Compensation
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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
Line Drop Compensation
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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
Line Drop Compensation
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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
Line Drop Compensation
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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
The measuredbus voltage
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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
Negative Reactance Control
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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
The regulatedvoltage
VREM
VREG
-ILX
-ILRIcR
-IcX
The reactance of LDC is reversed
VREG will be regulated correctlyreducing the over voltage
Negative Reactance Control
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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)
The measuredbus voltage
The regulatedvoltage
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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