slide 1 line current differential application on short lines presentation to sscet october 26 th,...
TRANSCRIPT
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Slide 1
Line Current Differential Application
on Short LinesPresentation to SSCET
October 26th, 2012
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• Goals of Protection
• Definition of Short Lines
• Challenges Posed by Short Lines
• Line Current Differential Explained
• Benefits of Line Current Differential
• Application Example
Content
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Goals of Protection
Security Dependability: the degree of certainty that the relay will operate correctly.Security: the relay will not operate incorrectly
Speed Very high power during fault conditions: delays translate into increased damage: faster protection tends to compromise relay system security and selectivity.
Sensitivity
The minimum operating quantities allows the relay to detect an abnormal condition. High-impedance ground faults, voltage unbalance and high source- to- line impedance ratio affect the sensitivity
Selectivity
or coordination: ability of the relay system to minimize outages as a result of a fault by operating as fast as possible within their primary zone.
Simplicity
simple to apply and to obtain maximum protection
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Slide 4
What is a short line?
Classification of line length depends on: Source-to-line Impedance Ratio
(SIR), and Nominal voltage
Length considerations: Short Lines: SIR > 4 Medium Lines: 0.5 < SIR < 4 Long Lines: SIR < 0.5
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Challenges of Short Lines
Sensitivity of Overcurrent Elements
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Challenges of Short Lines
Coordination of Distance Elements
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Challenges of Short Lines
Operation Time of Distance Elements
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Distance Relay Basics
For internal faults:
• IZ – V and V approximately in phase (mho)
• IZ – V and IZ approximately in phase (reactance)
RELAY (V,I)
IntendedREACH point
Z
F1
I*Z
V=I*ZF
I*Z - V
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Distance Relay Basics
For external faults:
• IZ – V and V approximately out of phase (mho)
• IZ – V and IZ approximately out of phase (reactance)
RELAY (V,I)
IntendedREACH point
Z
I*Z
V=I*ZF
I*Z - V
F2
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Distance Relay Basics
-0.5 0 0.5 1 1.5-100
-80
-60
-40
-20
0
20
40
60
80
100
Volta
ge [V
]
-0.5 0 0.5 1 1.5-3
-2
-1
0
1
2
3
4
5
Curr
ent [
A]
vA vB vC
iA
iB, iC
-0.5 0 0.5 1 1.5-100
-50
0
50
100
Reacta
nce c
om
para
tor
[V]
power cycles
SPOL
SOP
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Distance Relay Basics
LineSystem
Relay
Voltage at the relay:SIRf
fVV
PULOC
PULOCNR
][
][
Consider SIR = 0.1
Fault location
Voltage (%)
Voltage change (%)
75% 88.24 2.76
90% 90.00 0.91
100% 90.91 N/A
110% 91.67 0.76
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Distance Relay Basics
Line
SystemRelay
Voltage at the relay:SIRf
fVV
PULOC
PULOCNR
][
][
Consider SIR = 30
Fault location
Voltage (%)
Voltage change (%)
75% 2.4390 0.7868
90% 2.9126 0.3132
100% 3.2258 N/A
110% 3.5370 0.3112
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Current Differential Relay Basics
• Unit Protection• Communications Channel
Required
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Current Differential Relay Basics
Clock Synchronization
Communication path
Initial clocks mismatch=1.4ms or 30°
8.33 ms
8.33 ms
8.33 ms
Store T1i-2=5.1
8.33 ms
t1 t2
Slow down
Relay 10
5.1
0
2.3
8.33
8.33 Send T2i-2=2.3
Send T1i-2=5.1
Capture T1i-2=5.1
8.33 ms
Send start bitStore T1i-3=0
Send start bitStore T2i-3=0
13.4310.53
Send T1i-1=16.66
Capture T2i-2=2.3
16.66
21.76
16.66
18.96
Send T2i-1=16.66
Store T2i-1=16.66Capture T1i=21.76
Store T2i-2=2.3
Store T1i-1=8.33Capture T2i=18.96
T2i-3=0T1i-2=5.1T1i-1=16.66T2i=18.96
a2=5.1-0=5.1b2=18.96-16.66=2.32=(5.1-2.3)/2== +1.4ms (behind)
T1i-3=0T2i-2=2.3T2i-1=16.66T1i=21.76
a1=2.3-0=2.3b1=21.76-16.66=5.11=(2.3-5.1)/2== -1.4ms (ahead)
Speed up
Relay 2
30°0°
Measure channel delay to shift local phasor by angle equal to the half of the round trip delay:
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Current Differential Relay Basics
Clock Synchronization
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Current Differential Relay Basics
Communications Channel Noise
window
time
A sum of squared differences between the actual waveform and an ideal sinusoid over last window is a measure of a “goodness of fit” (a measurement error)
The goodness of fit is an accuracy index for the digital measurement
The goodness of fit reflects inaccuracy due to:• transients• CT saturation• inrush currents and other
signal distortions• electrical noise
The goodness of fit can be used by the relay to alter the traditional restraint signal (dynamic restraint) and improve security
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Current Differential Relay Basics
Traditional vs. Adaptive Restraint Differential
0
4 8 12
Irem pu
OPERATE
RESTRAINT
BP=8, P=2, S1=30%, S2=50%
BP=4, P=1, S1=30%, S2=50%
BP=4, P=1, S1=20%, S2=40%
OPERATE
Iloc pu
16 20
0
4
8
10
16
20
Pickup
Restraint 1
Restraint 2
Traditional characteristic
s
Adaptive characteristics
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Current Differential Relay Basics
Adaptive Restraint Differential
Total restraint = Traditional restraint + Adaptive restraint (Error factor)
Imaginary (ILOC/IREM)
Real (ILOC/IREM)
OPERATE
REST.
Error factor is high
Error factor is low
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Summary
• SIR, not just line impedance, defines a short line.• Overcurrent protection is less secure than
alternatives.• The sensitivity and speed of distance relaying are
adversely impacted, and coordination becomes more complex.
• Line current differential provides good sensitivity, speed and alleviates coordination issues.
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Application Examples
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Summary
51
51
51
51
51 51
SUB A
SUB B
SUB C
SUB D
SUB E
time
current
51 51
BLUE relay sees the most current.Coordination time intervals are acceptable.If line between Sub B and Sub C are out of service,coordination time interval between D and C is unacceptable.
87L 87L
By eliminating one of the 51 elements, we have increased the coordination time interval and made system coordination easier.
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Application Example
52
52
500 kV
230 kV
ZS = 0.01 pu
500 kV ZS = 0.02 pu
ZS = 0.01 pu
ZL = 0.003 pu ZL = 0.013 pu
ZL = 0.01 pu50 miles
14 miles 62 miles
SIR = 3.33
SIR = 6.67 SIR = 1.54
SIR = 0.76
Short line, weak source
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Application Example
Protection Scheme Needs
• High speed operation
• Weighted towards security
• Must protect short line without over-reaching
• Ability to handle weak source
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Application Example
POTT Scheme
52 52
RO 85RTransmit
ReceiveReceive
Trip CB
RO85RReceive
Receive
Trip CB
Transmit
RO
RO
• Plus: good security, distance relay, simple comms
• Minus: Communications channel, weak infeed conditions
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Application Example
Hybrid POTT
52 52
RO
Transmit
Receive
Receive
Trip CB
RO
RO
RU B
RUB
WI
RU
B
85R
0
T
Receive
Echo
Transmit
RO
WI
RU
B
This endidentical
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Application Example
Line Differential52 52
R
Trip CB Trip CB
RCVR
XMTR
Local +RemoteCurrent
R
RCVR
XMTR
Local +RemoteCurrent
• Plus: good security, good for short lines
• Minus: Complex communications channel
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Slide 27
References
• IEEE C37.113 Guide for Protective Relay Applications to Transmission Lines (1999) (draft 2011)Draft contains new information regarding short lines.
• Relaying Short Lines (Alexander, Andrichak, Tyska)GE Publication GER-3735.
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Questions