G. Ziegler. Digital Differential Protection
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G. Ziegler, 10/2005 page58
Differential Protection Symposium
Belo HorizonteNovember 2005
I
Diff
>
>
I
Diff
>
I
Res.
I
Op
I
Op
&
Trip
I
Op
>
≥1
Trip
I
Op
>>
Sampled
momentary values
i
Res
= | i
1
| + | i
2
|
Measured value
processing
Side 1
i
1L
i
2L
Side 2
i
Op
= i
1
+ i
2
87 algorithm
Operating characteristic
Harmonic Analysis:
-2nd Harmon. Blocking
-Cross Blocking
Generator, Motor and Transformer protection: Adaptive algorithmsto
upgrade relay stability and dependability with CT saturation (7UT6)
Motor start
DC component
I
Diff
>>>
i
Op
For dependability:
Fast tripping using sampled momentary values
ensures fast operation with very high currents before
extreme CT saturation occurs!
For security:
Adaptive restraint
I
Op
= RMS(i
Diff
)
60Hz
I
Res
= Mean(i
Res
)
Saturation detector
Mean values
Fundamental wave
G. Ziegler, 10/2005 page46
Differential Protection Symposium
Belo HorizonteNovember 2005
Current Transformers for Differential Relaying
Requirements and Dimensioning
Gerhard Ziegler
G. Ziegler, 10/2005 page47
Differential Protection Symposium
Belo HorizonteNovember 2005
2
1
12
N
N
⋅=
′
II
R
1
jX
1
jX
2
R
2
I
1
I
2
Z
b
S2P2
Ideal transformer
S1P1
I
m
N
1
N
2
Z
m
U
2
X
1
= Primary leakage reactance
R
1
= Primary winding resistance
X
2
= Secondary leakage reactance
Z
0
= Magnetising impedance
R
2
= Secondary winding resistance
Z
b
= Secondary load
Note: Normally the leakage fluxes X
1
and X
2
can be neglected
Equivalent current transformer circuit
G. Ziegler, 10/2005 page48
Differential Protection Symposium
Belo HorizonteNovember 2005
I
2
δ
I
1
I
m
.
w
1
w
2
I
1
ε
I
1
I
2
w
1
:w
2
j·X
2
R
2
Z
B
I‘
1
=
w
1
w
2
·I
1
I
2
R
2
R
B
U
2
U
2
I
m
X
m
Current transformer:
Phase displacement (δ) and current ratio error (ε)
E
m
I
m
E
m
G. Ziegler, 10/2005 page49
Differential Protection Symposium
Belo HorizonteNovember 2005
Dimensioning of CTsfor differential protection
CT classes to IEC 60044-1: 5P or 10P
Specification: 300/1 A 5P10, 30 VA
5% accuracy
at I= n x In
Accuracy
limit factor ALF
Rated burden
(nominal power) P
N
Ratio I
n -Prim
/ I
n -Sek.
Bi
Ni
'
PP
PP
ALFALF
+
+
×=
Actual accuracy limit factor
in operation is higher as the CT
is normally under-burdened :
Operating ALF: ALF‘
Dimension criterium:
TF
K
n
I
SC
I
ALF ×
−
≥
max
'
K
TF
(over-dimensioning factor) considers the single sided CT over-magnetising due to the d.c.
component in short circuit current I
SC
.
K
TF
values required in practice depend on relay type and design.
Recommendations are provided by manufacturers (see Application Guide)
R
CT
≤ 5 Ohm
CT
2
.seci
RIP ×=
G. Ziegler, 10/2005 page50
Differential Protection Symposium
Belo HorizonteNovember 2005
Current transformer, Standard for steady-state performance
Accuracy class
Current error
at nominal
current (In)
Angle error δ
at rated current
In
Total error at n x In
(rated accuracy limit)
5P
10P
± 1 %
± 5%
± 60 minutes
5 %
10 %
IEC 60044-1 specifies the following classes:
G. Ziegler, 10/2005 page51
Differential Protection Symposium
Belo HorizonteNovember 2005
Current transformers, Standard for transient performance
IEC 60044-6 specifies four classes:
Class
Ratio errorAngle error
Maximum error at
rated accuracy limit
TPX
(closed iron core)
TPY
with anti-remanence
air-gap
TPZ
linear core
± 30 min
± 30 min
± 180 ± 18 min
± 0,5 %
± 1,0 %
± 1,0 %
%
10
ˆ
≤
ε
only)current (a.c.
%
10
ˆ
≤
ε
%
10
ˆ
≤
ε
Error at rated current
TPS
closed iron core
Special version for high impedance protection
(Knee point voltage, internal secondary resistance)
Remanence
no limit
< 10 %
negligible
No limit
G. Ziegler, 10/2005 page52
Differential Protection Symposium
Belo HorizonteNovember 2005
U
KN
10 %
50 %
U
2
I
m
Definition of the CT knee-point voltage (BS and IEC)
British Standard BS3938: Class X
or
IEC 60044-1 Amendment 2000/07:
Class PX
Specify:
Knee point voltage
Secondary resistance R
CT
CTn
.maxSC
connectedBCTTFKN
I
I
)RR(KU
−
−
−
⋅+⋅≥
G. Ziegler, 10/2005 page53
Differential Protection Symposium
Belo HorizonteNovember 2005
(
)
()
CTratedB
CTANSIal
R520Z520
R520UE
⋅⋅+⋅⋅=
⋅
⋅
+
≈
−
ANSI C57.13 specifies:
•Secondary terminal voltage U
2
at 20 times rated current (20x5=100 A) and
rated burden
•Error <10%
Example:
800/5 A, C400 (R
B
= 4 Ω)
R
B
U
2
I
m
I‘
1
=
w
1
w
2
·I
1
I
2
R
CT
U
2
,
E
m
I
m
E
m
CT specification according to ANSI C57.13
Resulting magnetising voltage:
G. Ziegler, 10/2005 page54
Differential Protection Symposium
Belo HorizonteNovember 2005
Transient saturation with offset current
Steady-state saturation with a.c. current
Current transformer saturation