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Copyright © SEL 2015Copyright © SEL 2015
Line Protection: From Phasors to Traveling Waves
Karl ZimmermanTechnical Support Director
Schweitzer Engineering Laboratories, Inc.
Example 100 Mile Transmission Line
Z1S = 2 Ω 88° Z1L1 = Z1L2 = 8 Ω 84° Z1R = 2 Ω 88°Z0S = 2 Ω 88° Z0L1 = Z0L2 = 24 Ω 80° Z0R = 2 Ω 88°
Z0M = 16 Ω 78°
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Focus for Today
• Benefits of faster line protection
• Limitations of present-day phasor-based protection
• Principles of time-domain protection
• New Time-Domain Line Protection Relay
Already Pretty Fast – Why Faster?
• Higher power transfers(investment dollars saved)
• Reduced equipment wear (generators and transformers)
• Improved safety
• Reduced property damage
• Improved power quality
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How Much Faster?
• Present-day relays
Based on phasors
Operate in 0.5–1.5 cycles
• Present-day breakers operate in 2 cycles
• Ultra-high-speed fault clearing
Consistent relay operating times
2 ms (TW) to 4 ms (differential equations)
Subcycle times from future dc breakers
Phasor-Based Protection Makes Sense
• Power systems were traditionally designed and modeled for steady-state operation at system frequency
• “Forcing functions” are at system frequency
• Instrument transformers are rated at system frequency
• CCVTs are band-pass devices
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Speed of Present-Day Relays
• Phasors represent steady state
• Determining steady state takes time
This is what we know if we trip in 0.5 cycles
Speed of Present-Day Relays
• Phasors represent steady state
• Determining steady state takes time
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Speed of Present-Day Relays
• Phasors represent steady state
• Determining steady state takes time
• Shorter windows are faster but less accurate
Line Protection Using POTT Scheme With 21/67 Elements or
87L Scheme
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Zr
Z
dZ
Zp
ZS1
Dynamic Expansion
Basic Directional Element (32) Principle
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Symmetrical Components for Single-Line-to-Ground Fault
Negative-Sequence
Impedance Used to Determine
Fault Direction
2 2
2measured 22
Re V • 1 Z1ANG•IZ
I
L2Z Angle
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Protection Scheme Trip Time for Comm. Scheme Using 21/67
m (per-unit distance to fault)
TP
TT
(cy
cles
at 6
0 H
z) 1.5
0.5
1.0
0 0.25 0.50 1.00.75
Phase-to-Ground Fault
Phase-to-Phase Fault
Two-Terminal Digital Line Current Differential (87L) Application
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Alpha Plane Used for 87L Scheme
Angle
Rad
ius
Protection Scheme Trip Time for 87L
m (per-unit distance to fault)
TP
TT
(cy
cles
at
60 H
z) 1.5
0.5
1.0
0 0.25 0.50 1.00.75
87L Average (phase to ground)
87L Average (phase to phase)
87L Minimum (phase to ground)
87L Minimum (phase to phase)
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Phasor and Time-Domain PrinciplesSimilarities and Differences
Algorithm Phasor-Based Differential Equations Traveling Waves
Spectrum 50 / 60 Hz 1 kHz 100 kHz
Filtering
Sampling 16–32 s/c 10 kHz 1 MHz
Line theory
Operating time ~ 1 cycle A few milliseconds 1 ms
Requirements for CTs and PTs
Low Moderate High
TD21 Underreaching
Communications-independent
Comm-Based Scheme Using 32 (directional) TD32 and TW32 for direction
Fast communications as a teleprotection channel
TW87 Current-only
Direct fiber as a channel (100 Mbps)
GPS-independent
Time Domain: Speed With Security
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• Full-scale 1 MHz sampling
• TWs processed every microsecond
• -quantities processed every 0.1 ms
• Protection logic runs every 0.1 ms
Time Domain Uses High-Speed Data
Traveling-Wave Principles
12
L R
m
tL tR
Lightning and Faults Launch Traveling Waves
L R1
m t – t v2
L Rm ℓ – m
2(ℓ – m)
2m
2ℓ – m
3(ℓ – m)
ℓ – m2m
m
3m2(ℓ – m)
Low Z Low Z
t0
tL
tR
Polarity of Waves
13
Deriving TW87 Operating Principles Three Scenarios to Consider
–20 –10 0 10 20 30 40–200
0
200
Vol
tage
(V
)
–20 –10 0 10 20 30 40–5
0
5
Cur
rent
(A
)
Time (ms)
External Fault Behind Local Relay Remote Terminal
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Vol
tage
(V
)C
urre
nt (
A)
External Fault Behind Local RelayLocal Terminal
TW fault information is contained in thefirst 1–2 milliseconds of the fault
TW87 Principle – External Fault (F1)
Line propagation time
m87 = 1 pu
Loca
l Cur
rent
(A
)
Rem
ote
Cur
rent
(A
)
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TW87 Is Secure for External Fault (F1)
IL (A) IR (A) IDIF (A) IRST (A) m87 (pu)
F1
A 1.41 0.75 0.66 2.16 1.0
B 0.40 0.40 0.01 0.80 1.0
C 0.38 0.38 0.00 0.76 1.0
Loca
l TW
(A
)
Rem
ote
TW
(A
)
–200 0 200 400 600 800 1000–5
0
5
10
Time (µs)0 200 400 600 800 1000
–5
0
5
10
TW87 Principle – Internal Fault (F2)
Line propagation time
Line propagation time
m87 < 1 pu
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IL (A) IR (A) IDIF (A) IRST (A) m87 (pu)
F2
A 1.22 0.76 1.98 1.22 0.4
B 0.51 0.39 0.90 0.51 0.4
C 0.54 0.38 0.92 0.54 0.4
TW87 Operates in 1.5 ms
Loca
l TW
(A
)
Rem
ote
TW
(A
)TW87 Principle – External Fault (F3)
Line propagation time
Line propagation time
m87 < 1 pu
Loca
l Cur
ren
t (A
)
Rem
ote
Cur
rent
(A
)
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L R
F3
IL (A) IR (A) IDIF (A) IRST (A) m87 (pu)
F3
A 0.92 0.53 1.45 1.70 0.3
B 0.31 0.27 0.58 0.74 0.3
C 0.30 0.28 0.57 0.72 0.3
TW87 Is Secure for External Fault (F3)
L F RtF tF
Local TW Remote TW
Trip in 1.2 ms
Time (µs)
321
972
TW87 Operating Time on a 117 km Line
75
18
Processing Is Fast TW87 Speed Depends on Line Length
TW
87 T
ime
(ms)
Incremental Quantity Directional Element (TD32)
19
Forward Faultv Is Opposite Polarity to Replica Current
Reverse Faultv Is Same Polarity as Replica Current
20
v and i Are of Opposite Polarities for Forward Faults
0 2 4 6 8 10–100
–50
0
50
100
–100
–50
0
50
100
Time (ms)
0 2 4 6 8 10–100
–50
0
50
100
–100
–50
0
50
100
Time (ms)
Incremental Quantity Distance Element (TD21)
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Fault at the Reach|vF| Is Equal to |vPRE|
Fault Within the Reach|vF| Is Greater Than |vPRE|
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Fault Beyond the Reach|vF| Is Lower Than |vPRE|
Competitive TestingTime-Domain vs. Phasors
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Vol
tage
(V
)C
urr
ent
(A
)
TD21 amd TD32 Operate Faster 230 kV, 159 km, Fault at 18%
FSCC2
C1
L
Line voltage
Relay voltageStep-down transformer
Compensating reactor
Extracting Polarity of the First Wave
24
TD21 and TW87 ElementsComplement One Another
SE
L-T
400L
Tim
e (m
s)
TD21 Operates in Less Than 8 ms for a 100 Ohm Fault
25
Copyright © SEL 2015Copyright © SEL 2015
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