ABB 615 series Technical Manual page 402

Section 4
Protection functions
396
Ungrounded
100
90
80
Rf = 500 ohm
70 Rf = 2500 ohm
Rf = 5000 ohm
60
Rf = 10000 ohm
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90100
Total ground fault current (A), Rf = 0 ohm
GUID-0696D6D3-3811-4E4B-BC7C-AF00A27B95C9 V1 EN
Figure 190: Influence of fault resistance on the residual voltage magnitude in 20 kV
ungrounded and compensated networks. The leakage resistance is
assumed to be 30 times larger than the absolute value of the capacitive
reactance of the network. Parallel resistor of the compensation coil is
assumed to be disconnected.
Example
In a 15 kV, 50 Hz compensated network, the maximum value for VG during the healthy
state is 10%×Vph. Maximum ground-fault current of the system is 100 A. The maximum
ground-fault current of the protected feeder is 10 A (Rf = 0 Ω). The applied active current
forcing scheme uses a 15 A resistor (at 15 kV), which is connected in parallel to the coil
during the fault after a 1.0 second delay.
Solution: As a pickup condition for the admittance-based ground-fault protection, the
internal residual overvoltage condition of 21YN is used. The Voltage pickup value setting
must be set above the maximum healthy-state VG of 10%×Vph with a suitable margin.
Voltage pickup value = 0.15 × Vn
According to Figure 189, this selection ensures at least a sensitivity corresponding to a
2000 ohm fault resistance when the compensation degree varies between 80% and 120%.
The greatest sensitivity is achieved when the compensation degree is close to full
resonance.
A ground-fault current of 10 A can be converted into admittance.
10 A
Y j 1 15 .
= ≈ ⋅
Fdtot
15 kV 3
GUID-3631BAB9-7D65-4591-A3D6-834687D0E03C V2 EN
A parallel resistor current of 15 A can be converted into admittance.
15
A
G 1 73 . mS
= ≈
cc
15 kV 3
GUID-4B7A18DE-68CB-42B2-BF02-115F0ECC03D9 V2 EN
Resonance, K = 1
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
Total ground fault current (A), Rf = 0 ohm
mS
1MAC059074-MB A
Over/Under - Compensated, K = 1.2/0.8
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
Total ground fault current (A), Rf = 0 ohm
(Equation 38)
(Equation 39)
615 series ANSI
Technical Manual