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Date Code 20011026
Applications
5-9
SEL-321/321-1 Instruction Manual
Zero-Sequence Compensation Factor and Non-homogeneous Angle Settings
Zero-Sequence Compensation Factor Setting
The relay uses two zero-sequence compensation factors. The Zone 1 ground distance element
has a dedicated zero-sequence compensation factor (k01) and the remaining zones of ground
distance protection use a separately set zero-sequence compensation factor (k0).
The zero-sequence compensation factor is set using the following formula:
k
0
= (Z
0
- Z
1
) / (3 • Z
1
)
Where:
Z
0
= Zero-sequence
impedance
Z
1
= Positive-sequence
impedance
For this example both zero-sequence compensation factors are set the same and are based upon
the protected line impedance.
k01 = k0 = [(247.95
Ω
∠
81.47°)-(78.03
Ω
∠
84
°
)] / (3 • 78.03
Ω
∠
84
°
) = 0.726
∠
-3.69
°
Non-homogeneous Angle Setting
The relay also has a non-homogeneous angle setting (T) which improves the response of the
quadrilateral ground distance element. This may be needed when the source impedances at
either end of the line and the line itself all have different impedance angles. This angle setting
compensates the reactance characteristic of the quadrilateral element to improve the element
accuracy.
To determine a setting for T, run single-phase to ground faults where potential overreach may be
a concern. These locations are typically at the end of the Zone 1 quadrilateral reach or at the
remote bus. Compare the total zero-sequence fault current angle at the fault location and the
fault current angle from the relay location. If the angles compare closely, the system is
considered homogeneous and compensation is not required.
For our setting example, critical fault locations are those at the end of the Zone 1 reach or remote
bus fault where Zone 1 overreach is a concern.
For a single-phase to ground fault at Bus C, the total zero-sequence fault current is 1474 A
∠
-82.7°. The zero-sequence current contribution from Breaker 3 is 150 A
∠
- 82.7
°
. Note that
the angle between the total fault current and the fault contribution from Breaker 3 is zero.
Therefore, the non-homogeneous compensation is not required and T is set to zero.
k01M = 0.726
0.726
0.726
0.726
k01A = ----3.69
3.69
3.69
3.69
k0M = 0.726
0.726
0.726
0.726
k0A = ----3.69
3.69
3.69
3.69
T = 0000
- Relay Elements and Other Settings Affected
All set ground distance elements.
Содержание SEL-321
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