Ct Saturation; Figure 38: Ct Saturation Technique - GE P4A Technical Manual

P54A/B/C/E
5

CT SATURATION

The multi-ended line differential solution has some benefits to CT saturation. This enhanced technique, however,
does not signal when a CT is saturated, instead it directly discriminates between internal and external faults, even
when the CT is severely saturated.
The technique is GE patented technique, which is achieved by the correlation of the local measured phase current
with the remote current from all ends. Therefore, it eliminates the requirement to detect CT saturation to inhibit
protection and it can simply discriminate between internal and external fault. Because the new technique can
discriminate between internal and external fault even during severe CT saturation scenario, the relay can now use
much lower CT requirements than the existing P540D techniques. The technique involves simple calculations and
can be easily implemented in the relay. Both the reliability and security of the differential protection are improved
by this new detection technique.
Basic Principle
The CT saturation technique is based on the following principle:
E00769

Figure 38: CT saturation technique

If we define IPos(n) is the sum of positive values of all terminals at the moment 'n', and INeg(n) is the sum of
●
negative values of all terminals at the moment 'n', that is:
i
I
∑
=
Pos k
k Pos
∈
I i
∑
=
Neg
k
k Neg
∈
Where, k is the number of terminals, Pos means the set of terminals which current value is positive, Neg
●
means the set of terminals which current value is negative.
The difference between internal and external faults
Based on the analysis above, the discriminative characteristics between internal and external fault are presented
as follows:
I is in reverse direction to I
●
Pos
|| I ( ) n I
+
Pos Neg
I is 90 degree difference with INeg when internal fault occurs, as in:
●
Pos
|| I ( ) n I
+
Pos Neg
Where || || means RMS of the enclosed equation.
P54xMED-TM-EN-1
when external fault occurs, regardless if the CT is saturated or not; as in:
Neg
( ) || || n I ( ) n I
< −
Pos Neg
( ) || || n I ( ) n I
= −
Pos Neg
Chapter 6 - Current Differential Protection
( ) || n
( ) || n
Ratio > R
I
THRES
POS
I
NEG
Ratio < R
THRES
External
Fault
Internal
Fault
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