Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 74 / 734

3-20 Line Current Differential, Distance, Out-of-Step, Overcurrent, Date Code 20011112
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
CT Saturation Considerations for Three-Terminal Protection
In the example shown in Figure 3.14 an external fault behind Source S on the three-terminal line
causes Terminals L and R to source equal fault current. The current flowing out of Terminal L
and out of Terminal R is half the magnitude of the current flowing into Terminal S. The
three-terminal logic selects the Terminal S current as the local current when forming the Alpha
plane ratio, because it is the current with the largest magnitude. The same selection occurs in all
three relays. If a CT at Terminal S saturates, and the resulting current magnitude drops to less
than half of the perspective value, then the current from Terminal S will no longer be the largest
of the three-terminal currents. In that case the relays select one of the other terminal currents as
the largest. Assume the relays select Terminal L as the largest current magnitude, and so use that
current as the local current. The remote current is then I
R
+ I
S
. The current magnitude at S has
decreased to less than half of the non-saturated value. The inflow current from Terminal R
dominates the combination of I
R
+ I
S
, producing a net inflow for the external fault.
S
M311L095
R
L
2 • I
F
I
F
I
F
Figure 3.14: External Fault on Three-Terminal Line With Equal Infeed from Two
Terminals
To prevent the relay misoperating on a three-terminal line, the CTs must be selected and applied
such that the fundamental magnitude does not drop to less than half the unsaturated case for an
external fault. This selection criterion is quantified by the following equation.
( )
1
R
X
50
Z I
B F
+
≤
Equation 3.2
where: I
F
= secondary fault current, per unit of nominal (e.g., I
F
sec. / 5)
Z
B
= per unit of rated secondary burden (e.g., Z
B
sec. / 8 for class C800)
X/R = system X/R ratio