Ground Directional Overcurrent Example; Series Compensated Lines - GE D90 Plus Instruction Manual

SERIES COMPENSATED LINES

Ground directional overcurrent example

Series compensated lines
648
To ensure operation of the element under such circumstances, the angle comparator uses
a polarizing voltage augmented by the negative-sequence current as shown in the
following equations. For the forward-looking element:
For the reverse-looking element:
In the above equations, ECA represents the forward element characteristic angle
(maximum torque angle) and Z
augmentation for forward and reverse fault is shown in the above figure. As long as the
offset impedance is less than or equal to 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.
Plus
Consider a D90 with the following input signals
Assume an offset impedance of 4 Ω, an ECA angle of 88°, and a limit angle of 90°. The
Plus
D90 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°
The following signals for the directional unit of the negative-sequence directional
overcurrent element are also calculated. For the forward element, we have:
S_pol = 11.8 V ∠20.2°
S_op = 1.37 V ∠20.2°
For the reverse element, we have:
S_pol = 11.87 ∠20.2°
S_op = 1.37 V ∠–160.0°
After comparing the angles, a solid forward indication is given.
Assume further a pickup setting of 0.25 A for both forward and reverse directions, and the
negative-sequence mode set for the overcurrent unit of the element. The relay calculates
the operating signal using the positive-sequence restraint:
The operating signal is greater than the pickup setting of 0.25 A. Therefore, the overcurrent
unit will pickup and the element will operate in the forward direction.
Faults on or in a close vicinity of series compensated lines may create problems for
distance protection.
represents the offset impedance. The effect of the
offset
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 15: THEORY OF OPERATION
Eq. 81
Eq. 82
Eq. 83
Eq. 84