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

9.3 GROUND DIRECTIONAL OVERCURRENT

9.3GROUND DIRECTIONAL OVERCURRENT
Consider the negative-sequence directional overcurrent element. As illustrated below, the negative-sequence voltage could
be low during internal fault conditions.
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) and 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 identification 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  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:
9
• 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:
9-16
(a)
V_2
UR
(c)
V_2 >
I_2 * Z_2line
UR
Figure 9–6: OFFSET IMPEDANCE AUGMENTATION
S_pol V_2
= –

S_op I_2 1 ECA
=
S_pol V_2
= –

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 Protection System
(b)
V_2
(d)
S_pol
I_2 x Z
-V_2
  
I_2 Z_offset 1 ECA
+

  
I_2 Z_offset 1 ECA
+


9 THEORY OF OPERATION

9.3.1 DESCRIPTION

S_op
S_pol
I_2 x Z
-V_2
ECA
I_2
S_op
-V_2 V_2
ECA
I_2

9.3.2 EXAMPLE

(EQ 9.5)
(EQ 9.6)
GE Multilin