GE L90

CHAPTER 10: THEORY OF OPERATION DISTANCE ELEMENTS
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 10-29
10
Z0M/Z1 Magnitude: 0
Directional Comparator limit angle: 75°
Assume that the following signals are injected to the relay:
V
A
= 64.71 V ∠0.0° (pre-fault)
V
A
= 25.43 V ∠–19.9°
V
B
= 80.22 V ∠–133.5°
V
C
= 77.33 V ∠135.7°
I
A
= 4.47 A ∠–107.8°
I
B
= 2.92 A ∠68.9°
I
C
= 2.93 A ∠–51.1°
Based on the entered setting the relay calculates:
K0 = 3.58 ∠–15.2°
K0M = 0
Z = 14.00 Ω ∠88°
Z
D
= 1 Ω ∠88°
Z
R
= 9.99 Ω ∠−2°
Z
L
= 4.99 Ω ∠178°
For the assumed steady-state injection the relay calculates:
VA_1 = 58.83 V ∠–2.1°
VA_1M = 64.71 V ∠0.0°
I_0 = 1.37 A ∠–68.2°
IA_2 = 1.37 A ∠–68.1°
–V_0 = 29.18 V ∠8.4°
(V
A
–V
B
)_1 = 93.35 V ∠32.0°
(V
A
–V
B
)_1M = 112.08 V ∠30.0°
I
A
–I
B
= 7.39 A ∠–109.1°
10.3.6.2 Mho phase A to ground element (before memory expires)
I
A
× Z + I_0 × K0 × Z + I
A
× K0M × Z – V
A
= 103.33 V ∠–3.9°
V
A
_1M = 64.71 V ∠0.0°
I
A
_2 × Z
D
= 1.37 V ∠19.8°
I_0 × Z = 19.11 V ∠19.8°
I_0 × Z
D
= 1.37 V ∠19.8°
• Overcurrent supervision: | 3 × I_0 | = 4.09 A > 3 A
• Mho difference angle = | –3.9° – 0° | = 3.9° < 75°
• Reactance difference angle = | –3.9° – 19.8° | = 23.7° < 75°
• Zero-sequence directional difference angle = | 19.8° – 0.0° | = 19.8° < 75°
• Negative-sequence directional difference angle = | 19.8° – 0.0° | = 19.8° < 75°
• Fault-type comparator difference angle = | 19.8° – 19.8° | = 0.0° < 50°
All four comparators and the overcurrent supervision are satisfied, so the MHO phase A ground element operates for this
fault.
10.3.6.3 Mho phase A to ground element (after memory expires)
After the memory expires, the relay checks the actual positive-sequence voltage and compares it with 10% of the nominal
voltage:
| V
A
_1 | = 58.83 V > 0.1 × 69.28 V