CVT Transients Revisited –
Distance, Directional Overcurrent,
and Communications-Assisted
Tripping Concerns
David Costello and Karl Zimmerman
Schweitzer Engineering Laboratories, Inc.
Copyright © SEL 2012
Basic CVT Structure
Line Voltage (EL)
C1
C2
Compensating
Reactor (LC)
ZB
Step-Down
Transformer (XFMR)
1
Relay
Voltage
Cross Section of a CVT
Primary Terminal
Oil Expansion Diaphragm
Diaphragm Housing
Spring Assembly
HV Lead of Capacitor
Capacitor Stack
Porcelain Insulator
Intermediate Voltage Lead
LV Lead of Capacitor
Bushing Assembly
Coverplate
Reactance Protective Gap
Sight Glass Oil Level Indicator
Secondary Terminal Box
Transformer
Seal Block Assembly
Oil Sampling/Drain Valve
Reactance
Figure Courtesy of Alstom Grid Inc.
Physical Construction of CVTs
Photo Courtesy of Alstom Grid Inc.
2
CVT Transient Causes Classic Zone 1
Distance Element Overreach
CVT Transient Response at
Zero Crossing
CVT Transient and CT Saturation Cause
Directional Element to Misoperate
3
CVT Transient Response When
Fault Clears
CVT Transient Occurs on Fault Clearing
When Unfaulted Voltages Rise
CVT Transient During Ringdown
After Fault Clears
4
Factors That Affect Transient
Response of CVT
• Fault point-on-wave
• CVT stack capacitance
• Ferroresonance suppression
• Magnitude and composition burden
• Step-down transformer turns ratio and
excitation current
Voltage (V)
CVT Response at Voltage Zero
100
80
60
40
20
0
–20
–40
–60
–80
–100
–2
CVT Transient
Ratio Voltage
–1
0
1
2
Time (Cycle)
5
3
4
CVT Response at Voltage Peak
CVT Transient
Voltage (V)
100
80
60
40
20
0
–20
–40
–60
–80
–100
–2
Ratio Voltage
–1
0
1
2
Time (Cycle)
3
4
Fundamental Voltage
Magnitude (V)
High-Capacitance CVT Response Is Better
10
9
8
7
6
5
4
3
2
1
0
Ratio Voltage
High-Capacitance CVT
Normal-Capacitance CVT
1
1.5
2
2.5
3
Time (Cycle)
6
3.5
4
Ferroresonance Suppression Circuit
Relay Voltage
L
C
R
AFSC
Step-Down
XMFR
Relay Voltage
GAP
Lf Rf
PFSC
R
Step-Down
XMFR
PFSC Is Less Distorted Than AFSC
Active
Passive
Voltage (V)
100
80
60
40
20
0
–20
–40
–60
–80
–100
–2
Ratio
–1
0
1
2
Time (Cycle)
7
3
4
Fundamental Voltage
Magnitude (V)
PFSC Follows Ratio Voltage
Better Than AFSC
10
9
8
7
6
5
4
3
2
1
0
Passive CVT
Ratio Voltage
Active CVT
1
1.5
2
2.5
3
Time (Cycle)
3.5
4
Impact of CVT Burden
• ANSI C93.1 defines burden as R&X
• Resistive burden does not store energy,
provides better transient response
• Inductive burden worsens transient
response, oscillates at low frequency
8
Impact of CVT Burden
• Microprocessor-based relays are lower
burden, mostly resistive
• Rare case exists where higher burden
improves directional element response
(i.e., transient is worse, but damped
more quickly)
Classic Case: Zone 1
2.5
X-ohm
2
Without CVT
Transient
1.5
1
With CVT
Transient
0.5
0
–0.5
0
0.5
1
R-ohm
9
1.5
2
CG Fault Produces CVT Transient
and Zone 1 Overtrip
Zone 1 Overreach
• Conditions
♦
High SIR
♦
Low-capacitance, AFSC CVT design
• Solutions
♦
Change CVT or composition of burden
♦
Reduce reach or delay Zone 1
♦
Use CVT detection logic
10
Calculate SIR
SIR 
V1FAULT  V1PREFAULT
I1FAULT  I1PREFAULT  • Z1LINE
Maximum Zone 1 Reach
Maximum Zone 1 Reach
Setting (pu)
PFSC Versus AFSC
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Passive CVT
Burden = 100 ohms
Active CVT
0
5
10
11
15
SIR
20
25
30
Reduce Overreach Using Resistive Burden
Maximum Zone 1 Reach
Setting (pu)
1
Resistive Burden
0.95
ANSI Burden
0.9
0.85
0.8
0.75
SIR = 10
Maximum Rated Burden = 200 VA
0
20
40
60
80
% Maximum Rated Burden
100
Improved Zone 1 Setting Strategy:
Apply Two “Zone 1s”
Zone 1 (ZR1)
ZR1_NEW
Instantaneous
Delayed
Relay
12
Apply CVT Detection Logic
Distance Calculation Stable
CVT Transient Detected
0
Logic Enabled by User Setting
Switch-Onto-Fault
1.5
Delay
Zone 1
Distance
Element
Breaker Open
Zone 1 Trips Due to CVT Transient
Without CVT Detection Logic
13
Zone 1 Impedance Calculation
Ohms (sec)
4
MABi
2
Z1P
0
–2
–4
0
1
2
3
4
5
6
Cycles
7
8
9
CVT Detection Blocks Zone 1
Until Transient Subsides
14
10
11
Impact on Directional Elements
3IR2
90°
ZR2
ZL2
Z L 2 Angle
3V2
0°
Z2R
Z2F
ZS2
3IS2
Reverse Fault Momentarily Picks Up
Forward During CVT Transient
15
Erratic Response of
Z2 Directional Element
Negative Sequence Dirn Element Calc
1010
Z2Rthrei
Ohms (sec)
55
Z2i
Z2Fthre i
Z2Rthre i
00
Z2i
–55
Z2Fthrei
1
2
3
4
5
6
7
8
9
10
11
1
9.
98
8
10
.0
05
10
.0
21
10
.0
38
10
.0
55
10
.0
71
10
.0
88
10
.1
05
10
.1
21
10
.1
38
10
.1
55
–1010 0
9.
97
i
RS
Time (sec)
Result: Permissive Trip Signal Incorrectly
Transmitted to Remote Terminal
Independent Relay at Substation
Captures CVT and Conventional VT
CVT
CVT
CVT
Unfaulted A-Phase
Voltages
Unfaulted
B-Phase Voltages
Faulted
C-Phase Voltages
CVT Transient Observed on Faulted
and Unfaulted Phase Voltages
16
12
13
14
15
Follow CVT Manufacturer
Recommendations
“When high speed directional relays are
energized from this device it is
recommended that the basic burden be
power factor corrected to 100% or slightly
leading, and that the device be loaded to its
full rating of 150 watts, by the addition of
parallel resistance if necessary.”
– Coupling Capacitor Potential Devices
DCB Scheme Overtrips Due to
Delayed Transmit of Block Signal
17
CVT Transient
Extends
Ringdown
Conventional VTs
CVTs
Impact of CVT Transients
on Reclosing
VA
VB
VC
V1Mag
V1Ang
VA VB VC
100
0
-100
100
V1Mag
75
50
25
0
V1Ang
100
0
-100
1
2
3
4
5
6
Cy cles
18
7
8
9
10
11
Zone 1 Trips on Reclose
Due to V1 Memory
2500
IA
IB
IC
IAMag
ICMag
VA
VB
VC
IBMag
IA IB IC
0
-2500
IAMag IBMag ICMag
2000
1000
0
100
VA VB VC
0
Digitals
-100
3
ZAB
0
1
2
3
4
5
1111
6
Cycles
7
8
111
9
10
11
12
COMTRADE Replay With V1 Memory
Reset Produces No Trip
IA
IB
IC
VA
VB
VC
2000
IA IB IC
1000
0
-1000
-2000
100
VA VB VC
50
0
-50
Digitals
-100
ZAB
0
1
2
3
4
5
6
Cycles
19
7
8
9
10
11
12
Monitor Aging CVTs
Debris Scattered Following a
Catastrophic CVT Failure
Conclusions
20
Larger CVT Transients
• Zero-voltage point-on-wave faults
• Low CVT capacitance
• AFSC
• High SIR
• Low transformer ratios
• High transformer excitation current
• Inductive and larger burdens
Zone 1 Overreach Solutions
• For older relays
♦
Disable Zone 1
♦
Time-delay Zone 1
♦
Restrict Zone 1 reach per guidelines based
on burden, SIR, and ferroresonance
suppression design
• For newer relays, enable CVT transient
detection logic
21
CVT Transients Affect
High-Speed Reclosing
• Polarizing memory voltage of distance elements
can be corrupted
• Simple solutions
♦
Extend open intervals
♦
Set fault detectors above load
• Advanced solutions
♦
Adjust the time constant of the memory filter
♦
Substitute zeros into the memory filter when
ringdown or terminal-open condition is detected
CVT Transients Affect Directional
Comparison Schemes
• Directional elements are generally secure
• For AFSC CVT designs
♦
Adhere to specific burden requirements from
CVT manufacturer
♦
Consider using extended carrier coordination
delays for DCB schemes
♦
Consider delaying the transmission of
permissive signals for POTT schemes
22
CVTs Should Be Monitored
• Older and aging CVTs can fail and
explode violently
• Synchrophasors and automated
metering checks
♦
Extend maintenance intervals
♦
More importantly, alarm for discrepancies
and improve personnel safety
Questions?
23