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8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]
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Coupling Capacitor VoltageTransformers TransientsFundamentals
Roger Hedding ABB Dousman,WI
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Not this Transient
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Agenda
Introduction
Capacitor Voltage Transformer Design
Transient Response
Transient Response Effect on DistanceRelays
Selection of Capacitor Voltage Transformer
Conclusion
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Why Capacitor Voltage Transformer and What isthe Problem ?
Voltage transformer needed to transform system
voltage to voltage usable to IED.
Wire wound voltage transformers not economical
above 138kV.Capacitor voltage transformers consist of
capacitors and inductors that can discharge and
cause problems
Not a problem for e/m relays.
Faster sol id state and microprocessor relays need
attention
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Capacitor Voltage Transformer Cross Section
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CVT Schematic
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CVT Equivalent Circuit
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Transient response
Ability to reproduce rapid changes in
the primary voltage.
The remaining secondary voltage after aspecific t ime due to a short circuit on
the primary voltage
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Transient response
Transient oscil lations in the secondary
Two components
High frequency component
600 4000 Hz
Damped out in 10 milliseconds
Low frequency component
2 15 Hz
Lasts much longer
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Capacitor Voltage Transformer Transient Response
Transient Response function of
Equivalent Capacitance
Voltage Tap Ferroresonant Suppression
Connected Burden
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Transient Response vs Capacitance
Early work by Rusnak and Sweetana vintage 1970s
345kV PCA-7 CCVT
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Transient Response vs Tap Voltage
Early work by Rusnak and Sweetana vintage 1970s
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Transient Response vs Capacitance and Tap V
Early work by Rusnak and Sweetana vintage 1970s
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Transient Response Vs. Ferroresonant Suppression
Early work by Rusnak and Sweetana vintage 1970s
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Transient Response vs Burden
Early work by Rusnak and Sweetana vintage 1970s
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What has changed in modern day CVT design ?
Higher tap voltage up to 13kV
Higher capacitance
Changes in Ferroresonant suppression
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Transient Response for Current Design
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CVT Design Choices
Under control of
Designer
Stack
Capacitance
Ferroresonant
Suppression
Tap voltage
Under control of
user
Burden
Stack capacitance
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CVT Transient Response Effect on DistanceRelays
Impedance relays rely on
accurate information of CTs
and PTs to calculateimpedance to fault.
CVT transients cause
erroneous information to be
presented to the relay Transients last a relatively
short period of time
Zone 2 and Zone 3 timers
are set much longer the
transient time period. Zone 2 and Zone 3
impedance relays not
effected by CVT Transient
Zone 1 relay with nointentional delay will be
effected
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Factors effecting impedance relay
Type of fault
Fault incidence angle
Source Impedance Ratio of
protected line
Load current magnitude and
direction
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Franklin and Horton Paper
How much does zone 1 setting need to
be pulled back so it wont overrreach due
to CVT transient ?
Used CVT with active Ferroresonant
suppression
Worst case
Phase phase fault
Heavily loaded line flowing toward
relay
Fault occurs at voltage zero
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Impedance trajectories for Ideal VT and CVT
Franklin and Horton Determining Distance Relay Zone 1 Reach Settings to Prevent CCVT Transient Overreach
Phase Phase fault
SIR = 5
Zline = 0.1+J1.0
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Maximum Reach to compensate for CVT Transient
Franklin and Horton Determining Distance Relay Zone 1 Reach Settings to Prevent CCVT
Transient Overreach for a particular relay
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Modern Microprocessor relays
Use low pass filters and counters to mitigate
effects of CVT transients
Security comes at price of speed
Relay operation may be delayed by up to 10msec
Some manufactures use CVT Transient
filtering as setting (yes/no) in case
conventional wire wound VTs are used.
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Capacitance Selection for CVT
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Recommend EMTP Studies
Several models of CVTs have been developed for use with
EMTP Program.
To understand how a particular CVT will respond
on a given power system recommend EMTP Study
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Conclusion
Reviewed CVT Design
For lower CVT Transient
High Capacitance
High Tap Voltage
Passive Ferroresonant Circuit
Lower Burden
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Conclusion
Due to long time delay settings Zone 2 and Zone
3 relays typically not effected by CVT Transient
Zone 1 relays with no intentional delay subject to
overreach as a result of CVT transient.
Factors influencing Zone 1 overreach
SIR
Line Loading
Type of Fault
Fault inception angle
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Conclusion
Modern relays employ filtering techniques to
lessen impact of CVT transient
For further questions on your system do
EMTP study
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Questions ?