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Slide 1
Line Current Differential Application
on Short LinesPresentation to SSCET
October 26th, 2012
• Goals of Protection
• Definition of Short Lines
• Challenges Posed by Short Lines
• Line Current Differential Explained
• Benefits of Line Current Differential
• Application Example
Content
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
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
Challenges of Short Lines
Sensitivity of Overcurrent Elements
Challenges of Short Lines
Coordination of Distance Elements
Challenges of Short Lines
Operation Time of Distance Elements
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
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
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
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
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
Current Differential Relay Basics
• Unit Protection• Communications Channel
Required
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:
Current Differential Relay Basics
Clock Synchronization
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
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
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
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.
Application Examples
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.
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
Application Example
Protection Scheme Needs
• High speed operation
• Weighted towards security
• Must protect short line without over-reaching
• Ability to handle weak source
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
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
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
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.
Questions