Ground Directional Overcurrent; Description; Example - GE D30 Instruction Manual

8 THEORY OF OPERATION

8.3GROUND DIRECTIONAL OVERCURRENT

Consider the negative-sequence directional overcurrent element. As illustrated below, the negative-sequence voltage could
be low during internal fault conditions.
(a)
V_2
UR
(c)
V_2 >
I_2 * Z_2line
UR
In order to ensure operation of the element under such circumstances the angle comparator uses a polarizing voltage aug-
mented by the negative-sequence current as per following equations:
• Forward-looking element:
• Reverse-looking element:
where: ECA = forward ECA angle (maximum torque angle); Z_offset = offset impedance
The effect of the augmentation for forward and reverse fault is shown in the figures above. As long as the offset impedance
is not higher than the negative-sequence line impedance the element will ensure correct and fast fault direction identifica-
tion for both forward and reverse faults. The same principle applies to the neutral directional overcurrent element.
Consider relay input signals as in the Distance Elements Analysis section and assume an offset impedance of 4 Ω and
ECA and limit angles of 88° and 90°, respectively. The relay calculates the following negative-sequence quantities:
V_2 = 6.39 V ∠–159.6°; I_2 = 1.37 A ∠–68.1°; I_1 = 2.94 A ∠–144.2°
and the following signals for the directional unit of the negative-sequence directional overcurrent element:
• Forward-looking element:
• Reverse-looking element:
After comparing the angles, a solid forward indication is given.
Assume further the pickup setting of 0.25 A for both forward and reverse directions, and the "Negative-sequence" mode
setting entered for the overcurrent unit of the element. The relay calculates the operating signal using the positive-
sequence restraint:
I_op = |I_2| – |I_1| / 8 = 1.003 A > 0.25 A.
The overcurrent unit will pickup and the element will operate in the forward direction.
GE Multilin
Figure 8–6: OFFSET IMPEDANCE AUGMENTATION
×
S_pol V_2 I_2 Z_offset
= – +
× ∠
S_op I_2 1 ECA
=
×
S_pol V_2 I_2 Z_offset
= – +
S_op I_2 1 ECA × ∠
= –
∠
S_pol 11.87 V 20.2°
=
∠
S_op 1.37 V 20.2°
=
S_pol 11.87 V 20.2° ∠
=
S_op 1.37 V ∠ 160.0 °
= –
D30 Line Distance Relay
8.3 GROUND DIRECTIONAL OVERCURRENT
(b)
S_op
I_2 x Z
V_2 -V_2
ECA
I_2
(d)
S_op
S_pol
I_2 x Z
-V_2
-V_2
ECA
I_2
× ∠
1 ECA
× ∠
1 ECA

8.3.1 DESCRIPTION

S_pol
V_2

8.3.2 EXAMPLE

8-15
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