ABB RELION REX640 Technical Manual page 482

Section 4
Protection functions
476
Example
In a 15 kV, 50 Hz compensated network, the maximum value for Uo during the
healthy state is 10%×Uph. Maximum earth-fault current of the system is 100 A. The
maximum earth 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 start condition for the admittance-based earth-fault protection, the
internal residual overvoltage condition of EFPADM is used. The Voltage start value
setting must be set above the maximum healthy-state Uo of 10%×Uph with a suitable
margin.
Voltage start value = 0.15 × Un
According to Figure 260, 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.
An earth-fault current of 10 A can be converted into admittance.
10 A
Y j 1 15 .
= ≈ ⋅
Fdtot
15 kV 3
GUID-67919319-74E4-459C-8B49-019DFFC40CC1 V1 EN
A parallel resistor current of 15 A can be converted into admittance.
15 A
G 1 73 . mS
= ≈
cc
15 kV 3
GUID-F07E193D-844C-414C-9819-433C49DDBB62 V1 EN
According to Equation 47, during an outside fault EFPADM measures the following
admittance:
Yo Y j mS
≈ − ⋅1 15 .
= −
Fdtot
GUID-AA6A8AB4-E4F6-405D-9EE5-5EF4DF6326B8 V1 EN
According to Equation 50, during an inside fault EFPADM measures the admittance
after the connection of the parallel resistor:
Yo Y Y
1 73 .
(
= + ≈
Bgtot CC
GUID-2A963D3F-EE2A-4376-B93C-F5A65475DB6E V1 EN
Where the imaginary part of the admittance, B, depends on the tuning of the coil
(compensation degree).
The admittance characteristic is selected to be the combined overconductance and
oversusceptance characteristic ("Box"-characteristics) with four boundary lines:
Operation mode = "Go, Bo"
mS
j B mS
)
+ ⋅
1MRS759142 C
(Equation 59)
(Equation 60)
(Equation 61)
(Equation 62)
REX640
Technical Manual