3-2
Line Current Differential, Distance, Out-of-Step, Overcurrent,
Date Code 20011112
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
Theory of Operation (patented)
Figure 3.2 helps understand how the phase and negative-sequence differential elements operate
for a two-terminal line. Three-terminal cases are described later, but all of the two-terminal
discussion applies to three-terminal cases. Figure 3.2 shows the Alpha plane, which represents
the phasor or complex ratio of remote (I
R
) to local (I
L
) currents. There is a separate Alpha plane
for every current (phase, negative-sequence, zero-sequence, etc).
M311L076
Re
I
R
I
L
( (
I
R
I
L
( (
Im
1
∠
180˚
∠
I
R
I
L
( (
External Faults
and Load Current
I
R
I
L
Figure 3.2: Alpha Plane Represents Complex Ratio of Remote-to-Local Currents
Arbitrarily assign current flowing into the protected line to have zero angle, and current flowing
out of the protected line to have angle 180 degrees. Five Amps of load current flowing from the
local to the remote relay produces an A-phase current of 5
∠
0° at the local relay, and 5
∠
180° at
the remote relay. The ratio of remote to local current is:
°
∠
=
°
∠
°
∠
=
180
1
0
5
180
5
I
I
AL
AR
v
v
°
∠
=
°
−
∠
°
∠
=
180
1
120
5
60
5
I
I
BL
BR
v
v
°
∠
=
°
∠
°
−
∠
=
180
1
120
5
60
5
I
I
CL
CR
v
v
Equation 3.1
On the A-phase Alpha plane, this plots one unit to the left of the origin, as shown in Figure 3.2.
The other two phases also reside at 1
∠
180° on their respective Alpha planes.
In fact, all through-load current plots at 1
∠
180° regardless of magnitude and regardless of angle
with respect to the system voltages. Likewise, an external fault has equal and opposite current at
the two line ends, and so external faults also plot at 1
∠
180°.
The SEL-311L Line Current Differential Relay surrounds the point 1
∠
180°on the Alpha plane
with a restraint region, as shown in Figure 3.3. The relay trips when the Alpha plane ratio travels
Summary of Contents for SEL-311L
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