Ct Saturation Detection - GE L90 Instruction Manual

8.1 OVERVIEW
Finally, the local adaptive restraint term is computed as follows, for each local current:
Another source of the measurement errors is clock synchronization error, resulting in a clock uncertainty term. The L90
algorithm accounts for two terms of synchronization error corresponding to:
• Raw clock deviation computed from time stamps. There are several effects that cause it to not track exactly. First, the
ping-pong algorithm inherently produces slightly different estimates of clock deviation at each terminal. Second,
because the transmission of time stamps is spread out over several packets, the clock deviation estimate is not up to
date with other information it is combined with. Channel asymmetry also contributes to this term. The clock deviation
computation is indicated in equation 8.15 as θ
and then average of absolute values is computed. If GPS compensation is used, then GPS clock compensation is sub-
tracted from the clock deviation.
• Startup error. This term is used to estimate the initial startup transient of PFLLs. During startup conditions, a decaying
exponential is computed to simulate envelope of the error during startup
The clock uncertainty is expressed as:
Eventually, the local clock error is computed as:
The local squared adaptive restraint is computed from all local current sources (1 to 4) and is obtained as follows:
2
( )
I
=
LOC_ADA_A
Current differential protection is inherently dependent on adequate CT performance at all terminals of the protected line,
especially during external faults. CT saturation, particularly when it happens at only one terminal of the line, introduces a
spurious differential current that may cause the differential protection to misoperate.
The L90 applies a dedicated mechanism to cope with CT saturation and ensure security of protection for external faults.
The relay dynamically increases the weight of the square of errors (the so-called 'sigma') portion in the total restraint quan-
tity, but for external faults only. The following logic is applied:
• First, the terminal currents are compared against a threshold of 3 pu to detect overcurrent conditions that may be
caused by a fault and may lead to CT saturation.
• For all the terminal currents that are above the 3 pu level, the relative angle difference is calculated. If all three termi-
nals see significant current, then all three pairs (1, 2), (2, 3), and (1, 3) are considered and the maximum angle differ-
ence is used in further calculations.
8
• Depending on the angle difference between the terminal currents, the value of sigma used for the adaptive restraint
current is increased by the multiple factor of 1, 5, or 2.5 to 5 as shown below. As seen from the figure, a factor of 1 is
used for internal faults, and a factor of 2.5 to 5 is used for external faults. This allows the relay to be simultaneously
sensitive for internal faults and robust for external faults with a possible CT saturation.
If more than one CT is connected to the relay (breaker-and-the half applications), the CT saturation mechanism is executed
between the maximum local current against the sum of all others, then between the maximum local and remote currents to
select the secure multiplier MULT. A Maximum of two (local and remote) is selected and then applied to adaptive restraint.
8-12
4
2
( ) (
I --- - SumSquares
=
1_ADA_A
N
. If 2 channels are used, clock deviation is computed for both channels
i
clock_unc clock_dev
=
( )
clock_unc
--------------------------------- -
CLOCK
=
A
9
2 2
⋅ ( ( ) ( )
18 I I
+
1_ADA_A 2_ADA_A
L90 Line Differential Relay
2
( ) )
I
–
1_A k ( )
1_MAG_A
start_up_error
+
2
2
⋅ ( ( ) ( )
I I
+
LOC_RE_A LOC_IM_A
2 2
( ) ( )
I I
+ + +
3_ADA_A 4_ADA_A
8 THEORY OF OPERATION
2
)
2
( ) )
I CLOCK
+
q_ADA_A A

8.1.17 CT SATURATION DETECTION

(EQ 8.23)
(EQ 8.24)
(EQ 8.25)
(EQ 8.26)
GE Multilin