GE D90 Plus - Ground Directional Overcurrent Example

648 D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
GROUND DIRECTIONAL OVERCURRENT THEORY CHAPTER 15: THEORY OF OPERATION
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:
Eq. 81
For the reverse-looking element:
Eq. 82
In these equations, ECA represents the forward element characteristic angle (maximum
torque angle) and Z
offset
represents the offset impedance. The effect of the augmentation
for forward and reverse fault is shown in the preceding figure. As long as the offset
impedance is less than or equal to the negative-sequence line impedance, the element
ensures correct and fast fault direction identification for both forward and reverse faults.
The same principle applies to the neutral directional overcurrent element.
Ground directional overcurrent example
Consider a D90
Plus
with the following input signals:
Eq. 83
Assume an offset impedance of 4 Ω, an ECA angle of 88°, and a limit angle of 90°. The
D90
Plus
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 also are 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 as follows:
Eq. 84
The operating signal is greater than the pickup setting of 0.25 A. Therefore, the overcurrent
unit picks up and the element operates in the forward direction.
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