10-36
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
CHAPTER 10: THEORY OF OPERATION
10
If installed at the location X, the relay uses the following input signals for its phase AB distance element:
V = V
AB
= 77.402 kV
∠
57.5° primary or 29.49 V
∠
57.5° secondary
I = I
A
– I
B
= 2.576 kA
∠
–27.6° primary or 42.93 A
∠
–27.6° secondary
And consequently it sees an apparent impedance of
Z
app
= V / I = 30.05
Ω
∠
85° primary or 0.687
Ω
∠
85° secondary
If applied at location H, the relay sees the following input signals:
Table 10-11: Relay input signals at location H
The relay is set as follows:
XFMR VOL CONNECTION
= “Dy11”
XFMR CUR CONNECTION
= “Dy11”
Consequently, the following signals are applied to the phase AB distance element:
Eq. 10-40
Eq. 10-41
This results in the following apparent impedance:
Eq. 10-42
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:
Eq. 10-43
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,
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.
VB
97.23 kV
∠
–53.4°
37.04 V
∠
–53.4°
VC
181.8 kV
∠
–150.0°
69.26 V
∠
–150.0°
IA
1.288 kA
∠
–27.6°
21.47 A
∠
–27.6°
IB
1.288 kA
∠
152.4°
21.47 A
∠
152.4°
IC
0
0
Input
Primary
Secondary
VA
7.584 kV
∠
–5.59°
69.95 V
∠
–5.59°
VB
6.269 kV
∠
–120.1°
54.52 V
∠
–120.1°
VC
7.751 kV
∠
125.5°
65.84 V
∠
125.5°
IA
16.976 kA
∠
–27.6°
10.61 A
∠
–27.6°
IB
33.952 kA
∠
152.4°
21.22 A
∠
152.4°
IC
16.976 kA
∠
–27.6°
10.61 A
∠
–27.6°
Input
Primary
Secondary
