G. Ziegler. Digital Differential Protection
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G. Ziegler, 10/2005 page 231
Differential Protection Symposium
Belo HorizonteNovember 2005
Coupler CB auxiliary contacts
connected:
•87-A „sees“external fault and
remains stable
•67-B correctly trips BB-B
87 B
87 A
BB-A
BB-B
7SS52: Special treatment of dead zone faults (3)
Two CTsin coupling bay, overlapping protection zones
Coupler CB auxiliary contacts connected to 7SS52:
Coupler currents are removed from 87-A and 87-B current
comparison with open coupler CB.
G. Ziegler, 10/2005 page 232
Differential Protection Symposium
Belo HorizonteNovember 2005
Coupler CB close command is detected by
bus protection (change of biaryinput
signal):
•Coupler current is immediately re-
included in the 87 current comparison
before CB contacts close.
•Selective tripping of BB-B by 87-B.
BB-A
BB-B
7SS52: Treatment of switch onto a faulted (earthed) bus
G. Ziegler, 10/2005 page 233
Differential Protection Symposium
Belo HorizonteNovember 2005
7SS52 : Integral breaker fail protection
With line fault and CB failure:
•Trip command of bay protection hangs on at BB protection
•Forced current reversal of the current of concerned bay after T-BF
•Zone selective tripping only of the concerned busbar
•Advantage: Reset time (overtraveltime) of bay protection does not matter
&
&
Fault
detection
Trip
T-BF
Trip
∆I
87
Bay
protection
Busbar
protection7
SS52
Current
reversal
G. Ziegler, 10/2005 page 234
Differential Protection Symposium
Belo HorizonteNovember 2005
•BF trip command to busbarprotection binary input
(for security: fault detection signal as second criterion)
•Distribution of trip commands via isolator replica to breakers of concerned
busbar
• èzone selective tripping of concerned busbar
&
Fault
detection
Selective
bus
trip
T-BF
trip
I>
&
7SS52
Bay (feeder) protection
External bay dedicated BF protection
Zone selective tripping via isolator replica of 7SS52 busbarprotection
Isolator
replica
G. Ziegler, 10/2005 page 235
Differential Protection Symposium
Belo HorizonteNovember 2005
Fail safe design: 3-out-of-3 decision per bay
∆I of check zone
calculated
from all samples
∆I of discriminative
zones, calculated from
even samples
Trip-relay
(per bay)
∆I of discriminative
zones, calculated from
odd samples
L+
L–
G. Ziegler, 10/2005 page 236
Differential Protection Symposium
Belo HorizonteNovember 2005
1 ms
Check zone
Even
Samples
Odd
Samples
dt
dI
S
dt
dI
S
dt
dI
S
Adaptive measurement (Booster circuit) (7SS5)
G. Ziegler, 10/2005 page 237
Differential Protection Symposium
Belo HorizonteNovember 2005
01020
1
1
01020
1
20
0
t
01020
1.5
1
0.5
0.5
1
1
01020
1.5
1
0.5
0.5
1
1.5
I1
I2
I
Res
=|I1|+|I2|
I
Op
= |I1+I2|
∆I = I
Op
-I
Res
I1
I2
01020
0.5
1
1.5
2
01020
0.5
1
1.5
k·I
Res
= k·(|I1|+|I2|)
Digital busbarprotection 7SS5, Measuring technique
External fault, symmetrical fault current
G. Ziegler, 10/2005 page 238
Differential Protection Symposium
Belo HorizonteNovember 2005
01020
1.5
1
0.5
0.5
1
1.5
)
01020
1
1
01020
1.5
1
0.5
0.5
1
1
)
01020
1
2
01020
1
2
I
Op
= |I1+I2|
∆I=I
Op
-I
Res
I1
I2
I1
I2
I
Res
=|I1|+|I2|
01020
0.5
1
1.5
Tripping!
k·I
Res
=k·(|I1|+|I2|)
Digital busbarprotection 7SS5, Measuring technique
Internal fault, symmetrical fault current
G. Ziegler, 10/2005 page 239
Differential Protection Symposium
Belo HorizonteNovember 2005
()
∫
∫
+==Φ )(2)(2 ti
B
R
i
Rte
()()()
2
2N
I
ˆ
ALF'
B
R
i
Rdt
180
o
sinx
2
I
ˆ
ALF'
B
R
i
Rdt
180
o
(t)
2N
iALF'
B
R
i
Rdt
180
0
(t)
2N
uALF'
max
Φ
OOO
⋅⋅⋅+=⋅⋅⋅⋅+=⋅⋅+=⋅⋅≅
∫∫∫
I
m
φ
φ
max
R
B
2
2N
I
ˆ
ALF'dt
180
o
sinx
2N
I
ˆ
ALF'dt
x
o
sinx
2F
I
ˆ
O
⋅⋅=⋅⋅⋅=⋅
∫∫
∫
⋅=
x
o
sinx
2
ALF'
2N
I
2F
I
2
1
5
180
O
90
O
54
O
∫
==
x
o
sinx
2
ALF'
2N
I
2F
I
OB
k
I
2
I
M
R
i
I
1
E
2
7SS5/6: Admissible over-burdening -Necessary dimensioning of
CTswith regard to symmetrical fault currents
G. Ziegler, 10/2005 page 240
Differential Protection Symposium
Belo HorizonteNovember 2005
I
Res
= |I1|+|I2|
01020
0.5
1
1.5
01020
1
20
0
t
I
K
x
sin x
F
I2 ⋅⋅
I
Op
= |I1+I2|
Condition for stability:
I
Op
< k·I
Res
sin x
F
I2k
F
I ⋅⋅⋅<
(from previous transparency)
k2
1
sinx
⋅
>
1‘
2
cosx1
2
x
o
sinx
2
n'
2N
I
2K
I
ü
−
===
∫
1
14 −⋅
>
OB
K
OB
K
k
or
TF
2
K
TF
K4
1
k
−⋅
>
K
OB
= CT over-burdening factor
K
TF
= 1/K
OB
= CT over-dimensioning factor
Required restraining factor k dependent on over-burdening
factor K
OB
(over-dimensioning factor K
TF
)