GE L90 Instruction Manual page 778

PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
Input Primary
VB 97.23 kV ∠–53.4°
VC 181.8 kV ∠–150.0°
IA 1.288 kA ∠–27.6°
IB 1.288 kA ∠152.4°
IC 0
If installed at the location X, the relay uses the following input signals for its phase AB distance element:
V = V = 77.402 kV ∠57.5° primary or 29.49 V ∠57.5° secondary
AB
I = I – I = 2.576 kA ∠–27.6° primary or 42.93 A ∠–27.6° secondary
A B
And consequently it sees an apparent impedance of
Z = V / I = 30.05 Ω ∠85° primary or 0.687 Ω ∠85° secondary
app
If applied at location H, the relay sees the following input signals:
Table 10-11: Relay input signals at location H
Input Primary
7.584 kV ∠–5.59°
VA
6.269 kV ∠–120.1°
VB
7.751 kV ∠125.5°
VC
IA 16.976 kA ∠–27.6°
IB 33.952 kA ∠152.4°
IC 16.976 kA ∠–27.6°
The relay is set as follows:
XFMR VOL CONNECTION
XFMR CUR CONNECTION
Consequently, the following signals are applied to the phase AB distance element:
This results in the following apparent impedance:
This value is a correct measure of the distance from the VT location to the fault. For relay location 2, this certainly includes
the positive-sequence impedance of the transformer:
Thus, 0.127 Ω ∠90° + 0.05779 Ω ∠85° = 0.1847 Ω ∠88.4° primary side or 2.569 Ω ∠88.4° on the secondary side.
This example illustrates how the relay maintains correct reach for fault behind power transformers. When installed at X,
10
the relay needs to be set to 0.687 Ω ∠85° secondary in order to reach to the fault shown in the figure. When installed at H,
the relay needs to be set to 2.569 Ω ∠88.4° to ensure exactly same coverage.
10-36
Secondary
37.04 V ∠–53.4°
69.26 V ∠–150.0°
21.47 A ∠–27.6°
21.47 A ∠152.4°
0
Secondary
69.95 V ∠–5.59°
54.52 V ∠–120.1°
65.84 V ∠125.5°
10.61 A ∠–27.6°
21.22 A ∠152.4°
10.61 A ∠–27.6°
= "Dy11"
= "Dy11"
CHAPTER 10: THEORY OF OPERATION
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
Eq. 10-40
Eq. 10-41
Eq. 10-42
Eq. 10-43