G. Ziegler. Digital Differential Protection
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G. Ziegler, 10/2005 page48
Differential Protection Symposium
Belo HorizonteNovember 2005
I
2
I
1
Σ I= |I
1
| + |I
2
|
∆I=|I
1
+I
2
|
Protection object
I
1
I
2
t=0t=10
t=20 ms
t=0
t=10
t=20 ms
External fault
Internal fault
Differential protection
CT saturation with internal and external faults
∆I
G. Ziegler, 10/2005 page49
Differential Protection Symposium
Belo HorizonteNovember 2005
Saturation detector:
Locus of ∆I/ΣI for external faults without and with CT saturation
1
2
345
6
7
9'
108
0
3'
4'
5'
6'
7'
8'
9
n
o
p
e
r
a
t
e
r
e
s
t
r
a
i
n
External fault
(ideal CTs)
External fault
with CT
saturation
n=0
n=10
n=20
)n(21)n(sRe
i)n(ii +=
)n(2)n(1
ii +
)n(1
i
)n(2
i
)n(2)n(1
ii +
)n(2)n(1)n(Op
iii +=
G. Ziegler, 10/2005 page50
Differential Protection Symposium
Belo HorizonteNovember 2005
50
0
50
100
Differential current caused by transient CT saturation (ext. fault)
with operating and restraint current of busbar protection 7SS5
I
2
I
1
I
Res.
= |I
1
| + |I
2
|
I
Op
=|I
1
+I
2
|
Differential current appears only every second half wave!
G. Ziegler, 10/2005 page51
Differential Protection Symposium
Belo HorizonteNovember 2005
Tripping logic of digital busbarprotection 7SS600/7SS5
with saturation detector (simplified)
2 150 ms
A
N
D
n = 2
O
R
Trip
1-out-of-1
Trip
3 ms
3 ms
[]
ms/Ak
dt
di
s
sRe
>
sReOp
iki
⋅
>
setOp
ii >
A
N
D
A
N
D
Trip
2-out-of-2
Saturation
detected
Transient
blocking
A
N
D
A
N
D
Adaptive restraint
G. Ziegler, 10/2005 page52
Differential Protection Symposium
Belo HorizonteNovember 2005
Transformer differential protection 7UT6:
Saturation detection and automatic increase stabilisation
Area of
add-on
restraint
Restrain
Trip
Internal
faults
G. Ziegler, 10/2005 page53
Differential Protection Symposium
Belo HorizonteNovember 2005
87
I
F
I
F
I
F
+I
1
+I
2
+∆I
I
1
I
2
∆I
Transient CT saturation
causes false differential currents
Wave shape similar to
transformer inrush current ?
G. Ziegler, 10/2005 page54
Differential Protection Symposium
Belo HorizonteNovember 2005
Adaptive restraint against CT errors (7SD52/61)
Error
%
10%
ALF’ •I
N-CT
ALF‘/ALF
N
•I
N-CT
Fault range
f
L
Load range
f
F
CT Error approximation (no-saturation)
Detection of CT saturation
(wave shape analysis)
G. Ziegler, 10/2005 page55
Differential Protection Symposium
Belo HorizonteNovember 2005
Adaptive 87 restraint (7SD52/61)
considers current CT-errors
I
Diff
= │I
1
+ I
2
│
I
Restraint
=ΣI
Error
= I
Diff>
+ ε
CT1
·I
1
+ f
Sat
· ε
CT2
·I
2
Current summation:
Max. error summation:
I
2
I
1
Max. error (ε)
without
saturation
Max. error (ε)
with
saturation.
I
1
I
2
External
Fault
0
0
I
Rest
I
Diff
I
Diff>
Trip Area
Block-Area
Trip level
Without
saturation
Trip level
With
saturation
G. Ziegler, 10/2005 page56
Differential Protection Symposium
Belo HorizonteNovember 2005
External fault
Increase of stabilisation after detection of saturation
(K1:iE= -K1:iL1)
Begin of saturation
Increase of
restraint
∆ Ι
Σ Ι
I
Restraint
I
∆
G. Ziegler, 10/2005 page57
Differential Protection Symposium
Belo HorizonteNovember 2005
t/s
-0,02-0,01-0,000,010,020,030,040,050,060,070,08
iL1/kA
-40
-30
-20
-10
0
10
20
t/s
-0,02-0,01-0,000,010,020,030,040,050,060,070,08
iL2/kA
-40
-30
-20
-10
0
10
20
t/s
-0,02-0,01-0,000,010,020,030,040,050,060,070,08
iL3/kA
-30
-20
-10
0
10
20
t/s
-0,02-0,01-0,000,010,020,030,040,050,060,070,08
Diff G-AUS
¼cycle data windows of fast charge
comparison
1 cycle data windows for phasor
comparison
Q
1/4
cycle
•Synchronized with fault inception
•Released by I
diff
>>
Q
1
Q
2
Q
3
Fast charge comparison supplement
speeds up phasorbased line differential protection (7SD52/61)