G. Ziegler. Digital Differential Protection
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No.:1
Differential Protection Symposium Belo Horizonte, 7 to 9 Nov. 2005
Digital Differential Protection
G. Ziegler
No.:2
Differential Protection Symposium Belo Horizonte, 7 to 9 Nov. 2005
Differential Protection: Discussion Subjects
Ø Mode of operation
Ø Measuring technique
Ø Current transformers
Ø Communications
Ø Generator and motor differential protection
Ø Transformer differential protection
Ø Line differential protection
Ø Busbardifferential protection
G. Ziegler, 10/2005 page1
Differential Protection Symposium
BeloHorizonteNovember 2005
Digital Differential
Protection
Principles and Application
Gerhard Ziegler
G. Ziegler, 10/2005 page2
Differential Protection Symposium
BeloHorizonteNovember 2005
Contents
Pages
Mode of operation3 -27
Measuring technique28 -45
Current transformers 46 -86
Communications87 -115
Generator and motor differential protection116 -122
Transformer differential protection123 -178
Line differential protection179 -216
Busbardifferential protection217 -247
7UT6 product features248 -264
G. Ziegler, 10/2005 page 3
Differential Protection Symposium
Belo HorizonteNovember 2005
Digital Differential Protection
Mode of operation
Gerhard Ziegler
G. Ziegler, 10/2005 page 4
Differential Protection Symposium
Belo HorizonteNovember 2005
Comparison protection -Principles
Absolute selectivity by using communications
A
B
A
B
communication
Sampled values
Phasors
Binary decisions
t
protection range
•Directional comparison
Exchange of YES / NO signals
(fault forward / reverse)
• Current comparison
•comparison of
momentary values or phasors
•| ∆I | = |I
A
-I
B
| > limiting value
I
A
I
B
| ∆I |
I
A
I
B
forward
forward
reverse
reverse
Relay
Relay
G. Ziegler, 10/2005 page 5
Differential Protection Symposium
Belo HorizonteNovember 2005
Protection Criterion “current difference“
(differential protection)
n Kirchhoff‘s law: II
1
+ I
2
+ I
3
+ ... I
n
I= I
diff
= 0;
Current difference indicates fault
n Security by through-current dependent restraint
|I1|+|I2|+ ... |In| = I
Res
n Characteristic:
n Absolute zone selectivity (limits: CT locations)
No “back-up“for external faults
n Differential protection: for generators, motors, transformers, lines and busbars
Trip
I
res
I
diff
protection
object
G. Ziegler, 10/2005 page 6
Differential Protection Symposium
Belo HorizonteNovember 2005
Current differential protection: Basic principle
Protection
object
∆I>
I
1
I
2
i
1
i
2
∆I=I
1
+ I
2
=0
Protection
object
∆I>
I
1
I
2
i
1
i
2
∆I=I
1
+ I
2
i
1
i
2
internal fault
external fault or load
G. Ziegler, 10/2005 page 7
Differential Protection Symposium
Belo HorizonteNovember 2005
∆I
∆I
L1
L2
L3
Galvanicallyconnected circuits must only be
earthed once!
CT circuits of digital relays are
segregated and must each be earthed !
L1
L2
L3
∆I
∆I
Differential protection: Connection circuit
Digital relays have integrated
numerical ratio adaptation !
Different CT ratios need to be adapted by auxiliary
CTs!
Traditional technique
Digital technique
G. Ziegler, 10/2005 page 8
Differential Protection Symposium
Belo HorizonteNovember 2005
∆I
∆I
L1
L2
L3
∆I
∆I
∆I
L1
L2
L3
∆I
Y d 5
Matching
transformer
Generator and transformer differential protection