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written consent of Alstom Grid Technical Institute. All rights reserved.
GRID
Technical Institute
On/Under Load Tap ChangingTransformer
Voltage Regulating Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control2
Contents
On Load Tap Changing Transformers
Voltage Regulation
Time Delays
Grading Between Voltage Levels
Line Drop Compensation
Parallel Operation
Distributed Generation
Supplementary Control Functions
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On/Under Load Tap Changing Transformer Voltage Regulating Control3
Voltage RegulationPurpose
Electrical plant is designed to operate within finite
voltage limits
To control the system voltage transformers with tap
changers are commonly usedOff Load manually changed
On Load automatically adjusted
Tap changers physically alter the transformer ratio and
hence the voltage
Automatic voltage regulating relay is used to control the
tap changer
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On/Under Load Tap Changing Transformer Voltage Regulating Control4
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/Under Load Tap Changing Transformer Voltage Regulating Control5
On Load Tap Changing Transformers
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On/Under Load Tap Changing Transformer Voltage Regulating Control6
On Load Tap Changing TransformersTypical Supply Network
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On Load Tap Changing TransformersTap-Outs and Windings
Tap-outs are normally on the HV winding
Tap-outs can be at the Line End or Neutral End
Causes variations in the impedance of the transformer
Transformer impedance calculated at the centre 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 transitions
Early tap changers were reactors, could be continuously
rated useful when mechanisms were unreliable
Resistor transition tap changers now used, reliable
stored energy drive mechanisms
Typical Operation ~75ms
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On Load Tap Changing TransformersDouble Resistor Flag Sequence
1
2
3
4
5
6
7
8+
R R
Selector Compartment
No load switching takes place here.
'In Tank' double compartment tapchangers, have selectors immersed
in the main transformer tank.
'External' double compartment tap
changers, have selectors external to
the main transformer tank andseparate from diverters, but can
share oil with the main transformer
tank.
Diverter Compartment
Oil contamination takes place here.Oil kept separate from selectors
and main transformer tank.
All load switching takes place here.
Double Compartment Tap Changer Single Compartment Tap Changer
Oil is shared between selectors and
diverters.
Oil is not shared between the
single compartment and the maintransformer tank.
The diverter may be a vacuum
switch to prevent oil contamination.
Double compartment tap changersare usually found on large
distribution and transmissiontransformers.
Single compartment tap changers
are usually found on distribution
transformers.
All switching takes place in the same
compartment.
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Voltage Regulation
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Voltage RegulationBasic Operation
Controller monitors secondary voltage
voltage setting - nominal voltage level
voltage deadband - permissible range
Initial Time delay used to filtertransient voltage fluctuations
definite time
inversely proportional to the
deviation from the voltage
setting
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Control a voltage
regulating
transformer (on-load
tap changer)
Maintain system
voltage within set
limits
Issue tap up/down
commands
Time
Voltage
Vs
+ dVs
- dVs
Basic Regulating Requirements
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Voltage RegulationExample Operation
+ Voltage
Deadband
- Voltage
Deadband
Voltage
Setting
Time Delay
t
V
Voltage out
of deadband
Voltage restored
to within deadband
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Voltage RegulationAlgorithms
There are different voltage regulation algorithms
Basic On/Off Controller
Simple time delayed with immediate reset
Hysteresis on the Voltage Limits
Using different pick-up and drop-off quantities
Proportional Resetting Initial Delay Timers
The timer resets proportionally
Complex
Standard deviation measures
Performance criteria functions
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Time Delays
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DeadbandVoltageV
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Initial DelayInter-tap Delay
Time
Tap pulse duration - 0 to 5s (MVGC fixed at 1s)
Multiple Tap Change SequenceInter-Tap Delay - Definite Time
VoltageDeviation
Vs
dVs
The timer is reset after each operation to invoke initial timer
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VoltageDeviation
Vs
dVs
Initial Delay (Inverse Time)
Multiple Tap Change SequenceInter-Tap Delay - Inverse Time
Time
The timer is reset after each operation to invoke initial timer
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Initial Time DelaysTypical Setting Ranges
Initial Time Delay Setting Ranges
0 to 300s
Inter-tap Delay Setting Ranges
0 to 120s (typically 5 10s)
Initial Time Delays also ensure time grading between
voltage levels
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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 three reasons
Change in downstream load
Change in upstream supply voltage
Embedded generation
di l l
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Grading Between Voltage LevelsTime Grading
Grid Supply
Load
Load
A
C
BT3
T2
T1
Load
Discrimination to allow higher voltage to operate first
T1 time delay < T2 time delay < T3 time delay (initial)
Otherwise hunting can occur between voltage levels
Can be compromised by embedded/distributed generation
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On/Under Load Tap Changing Transformer Voltage Regulating Control23
Line Drop Compensation
Li D C ti
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On/Under Load Tap Changing Transformer Voltage Regulating Control24
Line Drop CompensationRegulating the Voltage at a Remote Point
Tap Changer
Time Delay
VoltageDeadband
Voltage SettingAVC Relay
Supply
Load
VT
CT
+-
Bus
X R
Feeder
Voltage Drop
Line Drop Compensation
)(FEEDERFEEDERLOADSLOAD
jXRIVV
)( AVCAVCAVCSBUS jXRIVV
Simulates voltage drop of the line
Artificially boosts transformer voltage at times of high loading
Li D C ti
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On/Under Load Tap Changing Transformer Voltage Regulating Control25
Line Drop CompensationRelay Implementation
Vr Vxl
Remote
Voltage
VREM
Load
IL
Bus Voltage VBUS = VREM + Vr + Vxl
Calculate the Voltage Drop at nominal load
Bus
Voltage
VB
Li D C ti
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On/Under Load Tap Changing Transformer Voltage Regulating Control26
Line Drop CompensationPhasor Diagram
IL
VBUS
-ILX
-ILR
VREM Voltage Setting
The relay calculates and
regulates the VREM magnitude
ILX required for power factor
The measured
bus voltage
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On/Under Load Tap Changing Transformer Voltage Regulating Control28
Parallel Operation
P ll l O ti
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Parallel OperationTransformer Operating in Parallel
Transformers in parallel can cause Circulating Currents
Circulating Current affects LDC causing tap changer
runaway
Circulating Current are damaging to Transformer
Therefore special control techniques are required
Paralleling using Master-Follower Control
Paralleling using Circulating Current Control
Paralleling using Negative Reactance Control
P ll l O ti
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On/Under Load Tap Changing Transformer Voltage Regulating Control30
Parallel OperationMaster - 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 critical
Circulating current should be monitored
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control31
Parallel OperationCirculating Currents due to Tap Disparity
2IL
T1
T2
Ic
IL-Ic
IL+Ic
If T1 is on a higher
tap than T2
Current seen by
#1 is IL + IC
Current seen by
#2 is IL - IC
#2
#1
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control32
Parallel OperationCirculating Current Effect on LDC
2IL
T1
T2
Ic
Vxl Vr
IL-Ic
IL+Ic
IC is measured in LDC circuit but
is not present in the feeder
#2
#1
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control33
Parallel OperationLDC on T1
IL
VBUS
-ILX
-ILR
VREM
ICIL + IC (Volts High)
VREG
-IcR
-IcX
The regulated voltage VREG
contains a -VC component
VREG will be regulated up to VREM,
therefore increasing the over
voltage even further
Voltage Setting
The regulatedvoltage
The measured
bus voltage
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control34
Parallel OperationLDC on T2
IL
VBUS
-ILX
-ILR
VREM
IC
IL - IC (Volts
Low)
VREG
-ICR
-ICX
The regulated voltage VREG
contains a +VC component
VREG will be regulated down to VREM,
therefore reducing the under
voltage even further
The measured
bus voltage
Voltage Setting
The regulated
voltage
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control35
Parallel OperationCirculating Current Control
Pilot Method (when tap changers are matched)
Extract the circulating current
Apply to a compensation circuit, derive a proportional
voltage magnitude
Feed this derived VC back into the measuring circuit to
compensate for the circulating current effect
Circulating Current Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control36
Circulating Current ControlPilot Method
T1
T2
VC
VC
T1>T2
Ic
jXt
jXt
IL+IC
IL-IC
IC
-IC
IL
IL
V1C = ICXt
V2C = -ICXt
2IL
Circulating Current Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control37
Circulating Current ControlCirculating Current Compensation on T1
IL
VBUS
-ILX
-ILR
VREM
ICIL + IC (Volts High)
VREG
Voltage Setting
VC
The voltage VREG is compensated
by VC
VREG will be regulated correctly
reducing the over voltage
The measured
bus voltage
The regulated
voltage
-IcR
-IcX
VREG
Circulating Current Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control38
Circulating Current ControlCirculating Current Compensation on T2
The voltage VREM is compensated
by VC
VREG will be regulated correctly
increasing the under voltage
IL
VBUS
-ILX
-ILR
IC
IL - IC (Volts
Low)
VREGVoltage Setting
VC
VREM
The measured
bus voltage
The regulated
voltage
VREG
-ICX
-ICR
Alternative Connections for
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On/Under Load Tap Changing Transformer Voltage Regulating Control39
Ic
IL+Ic IL-Ic
Ic
IL IL
IcRs Rs
IL
+Ic -Ic
2IL
Requires the use of swamping resistors (Rs)
Alternative Connections forLDC Circuits (Parallel)
Alternative Connections for
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On/Under Load Tap Changing Transformer Voltage Regulating Control40
Ic
IL+Ic
IL-Ic2IL 2IL
IL
+Ic -Ic
IL+Ic IL+Ic
ICTICT
2IL
Requires the possible use of interposing CTs (ICTs)
Alternative Connections forLDC Circuits (Series)
Parallel Operation
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On/Under Load Tap Changing Transformer Voltage Regulating Control41
Parallel OperationNegative Reactance Control
Reverse Reactance Control (non matched tap changers
or different sources)
Utilises the reactance compensation of the Line Drop
Compensation circuit
Reverses reactance and feeds back into compensation
circuit
Lack of specific feeder reactance in LDC results in
susceptibility to power factor
Negative Reactance Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control42
Negative Reactance ControlCompensation on T1
IL
VBUS
VREM
ICIL + IC (Volts High)
VREG
Voltage Setting
ILXt
-ILR-ICRICXt
The measured
bus voltage
The regulated
voltage
VREM
VREG
-ILX
-ILR-IcR
-IcX
The reactance of LDC is reversed
VREG will be regulated correctly
reducing the over voltage
Negative Reactance Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control43
Negative Reactance ControlCompensation 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 correctly
increasing the under voltageIL - IC (Volts
Low)
The measured
bus voltage
The regulatedvoltage
VREM
VREG
Negative Reactance Control
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Negative Reactance ControlUnity Power Factor
IL
Error
VBUSVREG
ILXt
VX = ILXt
VR = 0
Small error present at
unity power factor
Negative Reactance Control
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Negative Reactance ControlNon - Unity Power Factor
IL
VBUSVREG
ILXt
VX = ILXt
VR = IL(Xt.tan) Error enlarges with a lower
power factor, unless R is
specified
ILXt.tan
Negative Reactance Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control46
Negative Reactance ControlLDC and Low Power Factor
ZA2390
IL
VBUS
ILXt
-ILR
VREM
-ILX
-ILR
VREG
IL(Xt+X).tan
Error
VX = ILXt
VR = -ILR + IL(X+Xt).tan Addition of LDC results in
greater error
Negative Reactance Control
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On/Under Load Tap Changing Transformer Voltage Regulating Control47
Negative Reactance ControlLDC and Low Power Factor Correction
IL
VBUS
ILXt
-ILR
VREM
-ILX
-ILR
VREG
IL(Xt+X).tan
VX = ILXt
VR = -ILRcos - IL(Xt+X).sin Corrects for low power
factor
VR
ILR
IL(Xt+X).tanVR
(IL(Xt+X).tan).cos = IL(X+Xt).sin
VREG
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Distributed Generation
Distributed Generation
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Distributed Generationand LDC circuits
Distributed generation can cause reverse power flow
through transformer bus Effects LDC circuit and hence voltage regulation
Bus
Voltage
VB
Load A
Load B
Load C
G
When:
G < Load A no significant problem (if design practice followed)
G > Load A potential for low voltage levels due to reduced observed load
G > Load A, B and C LDC scheme will not operate correctly due to reverse power
Distributed Generation
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On/Under Load Tap Changing Transformer Voltage Regulating Control50
Distributed Generationand Cancellation CT
Cancellation CT can be used to maintain direction load
seen by control relayMaintains voltage during high load periods
VB Load A
Load B
Load C
G
Now When:
G < Load A no significant problem (if design practice followed)
G > Load A IG summed into IB therefore control relay sees all load current
G > Load A, B and C LDC scheme will not operate correctly due to reverse power
IB
IG
Distributed Generation
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On/Under Load Tap Changing Transformer Voltage Regulating Control51
Distributed Generationand Negative Reactance Schemes
Distributed generation can cause a change in power factor
This will effect Negative/Reverse Reactance paralleling
schemes which require a constant and specified power
factor
Circulating current minimisation schemes are preferential
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Supplementary Control Functions
Supplementary Control Functions
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pp y
Load Shedding
voltage 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 Functions
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pp yTap Changer Supervision
Tap Changer Monitoring
tap changer confirmation response (time limited)
voltage monitoring
voltage change occurs
in the correct direction
of sufficient magnitude
operations with no initiating signal
Tap Change Operations Counter
alarms at settable threshold
frequency alarms
excessive operations per time period
(day / month / maintenance period)