On/Under Load Tap Changing Transformer Automatic Voltage Regulating Control Relay
Contents
X On Load Tap Changing Transformers X Voltage Regulation X Time Delays X Grading Between Voltage Levels X Line Drop Compensation X Parallel Operation
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Voltage Regulation Purpose
X Electrical plant is designed to operate within finite voltage limits X To control the system voltage transformers with tap changers are commonly used
Off Load – manually changed On Load – automatically adjusted X Tap changers physically alter the transformer ratio and hence the voltage X Automatic voltage regulating relay is used to control the tap changer
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Voltage Regulation Performance Criteria
X Two Mutually Exclusive Parameters
voltage quality number of tap changes X Voltage Quality
customer focused quality measurement percentage of time outside deadband or statuary limits length of voltage disturbance X Number of Tap Changes
supplier focused quality measurement directly relates to cost of ownership of the tap changer 5
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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On Load Tap Changing Transformers
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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On Load Tap Changing Transformers Typical Supply Network
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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On Load Tap Changing Transformers Tap-Outs and Windings
XTap-outs can be in the HV or LV winding XTap-outs can be at the Line End or Neutral End XCause a variation in the impedance of the transformer
Transformer impedance calculated at the centre tap position
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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On Load Tap Changing Transformers Development of the Tap Changer XDeveloped in the late 1920s XTwo primary considerations
The load current must not be interrupted No windings should be short-circuited during transitions XEarly tap changers were reactors, could be continuously rated – useful when mechanisms were unreliable XResistor transition tap changers now used, reliable stored energy drive mechanisms
Typical Operation ~75ms 9
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Voltage Regulation
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Voltage Regulation Basic Operation X Controller monitors secondary voltage
voltage setting - nominal voltage level voltage deadband - permissible range X Initial Time delay used to filter transient voltage fluctuations
definite time inversely proportional to the deviation from the voltage setting
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Voltage Regulation Example Operation
V Voltage out of deadband
Time Delay
+ Voltage Deadband Voltage Setting - Voltage Deadband
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
t
Voltage restored to within deadband
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Time Delays
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Time Delays Initial Time Delays X Initial time delay used to filter transient voltage fluctuations
definite time inversely proportional to the deviation from the voltage setting Operating Time
Definite Time Characteristic
Inverse Time Characteristic
Voltage VBand+
VSetting
VBand-
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Multiple Tap Change Sequence Inter-Tap Delay - Definite Time
Voltage Deviation Initial Delay
Initial Delay (Definite Time)
+dVs Vs
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
Time
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Multiple Tap Change Sequence Inter-Tap Delay - Inverse Time
Voltage Deviation
Initial Delay (Inverse Time)
dVs Vs
Time
X The timer is reset after each operation to invoke initial timer
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Initial Time Delays Typical Setting Ranges
X Initial Time Delay Setting Ranges
MVGC - 0 to 120s KVGC - 0 to 300s X Inter-tap Delay Setting Ranges
MVGC - 0 to 10s KVGC - 0 to 120s X Initial Time Delays also ensure time grading between voltage levels
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Grading Between Voltage Levels
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Grading Between Voltage Levels Causes of Voltage Deviations Load T1 Line
Grid Supply A
T2
B Load
T3
C Load
X Voltage deviations occur for two reasons
Change in downstream load Change in upstream supply voltage
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Grading Between Voltage Levels Time Grading Load Load T1
Load
Grid Supply
T2
A T3
B C
X Discrimination to allow higher voltage to operate first
T1 time delay < T2 time delay < T3 time delay (initial) X Otherwise hunting can occur between voltage levels
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Line Drop Compensation
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Line Drop Compensation Regulating the Voltage at a Remote Point X Simulates voltage drop of the line X Artificially boosts transformer voltage at times of high loading Bus
Supply Feeder
CT
Tap Changer
Load X R Voltage Drop
VT
Time Delay
Voltage Deadband AVC Relay
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+ -
Line Drop Compensation
VLOAD = VS − I LOAD(RFEEDER + jX FEEDER) VBUS = VS − I AVC (RAVC + jX AVC )
Voltage Setting
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Line Drop Compensation Relay Implementation X Calculate the Voltage Drop at nominal load
Vr
Vxl
Load IL
Bus Voltage VB
Remote Voltage VREM
Bus Voltage VBUS = VREM + Vr + Vxl 23
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Line Drop Compensation Phasor Diagram
-ILX -ILR
The measured bus voltage
VBUS VREM
IL
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Voltage Setting
X The relay calculates and regulates the VREM magnitude
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Line Drop Compensation Typical Setting Ranges
XReactive and Resistive Compensation, Vr and Vxl
MVGC - 0 to 48V KVGC - 0 to 50V
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Transformers Parallel Operation
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation Transformer Operating in Parallel X Transformers in parallel can cause Circulating Currents
Circulating Current affects LDC causing tap changer runaway
Circulating Current are damaging to Transformer X Therefore special control techniques are required
Paralleling using Master-Follower Control Paralleling using Negative Reactance Control Paralleling using Circulating Current Control 27
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation Master - Follower
X One controller designated the master X All other controllers (followers) follow the master X Requires that
Identical transformers / tap changers Locality of equipment Security of communications is critical X Circulating current should be monitored
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation Circulating Currents due to Tap Disparity
#2
T2
X If T1 is on a higher tap than T2
IL-Ic
2IL Ic
T1
IL+Ic
X Current seen by #1 is IL + IC X Current seen by #2 is IL - IC
#1
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation Circulating Current Effect on LDC
#2
T2
IL-Ic
Vxl Ic
T1
Vr
2IL
IL+Ic #1
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X IC is measured in LDC circuit but is not present in the feeder
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation LDC on T1 -ILX -IcR
The measured bus voltage
The regulated voltage
-ILR
-IcX VREM VBUS VREG IL
IC IL + IC (Volts High)
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Voltage Setting
X The regulated voltage VREG contains a -VC component X VREG will be regulated up to VREM, therefore increasing the over voltage even further
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation LDC on T2 -ILX -ILR
The measured bus voltage
-ICX -ICR
VREM
The regulated voltage
VBUS VREG
Voltage Setting
IL IC IL - IC (Volts Low)
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X The regulated voltage VREG contains a +VC component X VREG will be regulated down to VREM, therefore reducing the under voltage even further
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Parallel Operation Negative Reactance Control
X 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
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Negative Reactance Control Compensation on T1 The regulated voltage
ICXt
-ICR
ILXt
-ILX
-ILR VBUS
VREG
-IcR
-ILR
-IcX V REM
VREM
The measured bus voltage Voltage Setting
VREG
IL
IC IL + IC (Volts High)
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X The reactance of LDC is reversed X VREG will be regulated correctly reducing the over voltage
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Negative Reactance Control Compensation on T2 ILXt
-ILX
-ILR-I R C
The measured bus voltage
VREM The regulated voltage
-ILR
-ICR
ICXt VBUS
VREG
VREG
VREM IL
IC IL - IC (Volts Low)
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-ICX
Voltage Setting
X The reactance of LDC is reversed X VREG will be regulated correctly increasing the under voltage
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Supplementary Control Functions AVR Relaying
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Supplementary Control Functions X Load Shedding
voltage setting override (± 10%) X Over Current / Under Voltage Supervision
inhibit control during fault conditions X Excessive Circulating Current
protects against parallel control failure X Under Current Inhibit
stops operations when low forward or reverse power X Reverse Current Operation X Auto/Manual/Remote Operations X Measurements
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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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Supplementary Control Functions Tap Changer Supervision X Tap Changer Monitoring
tap changer confirmation response (time limited) voltage monitoring z
voltage change occurs
z
in the correct direction
z
of sufficient magnitude
operations with no initiating signal X Tap Change Operations Counter
alarms at settable threshold frequency alarms z
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excessive operations per time period (day / month / maintenance period)
On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays
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