ABB RELION Series, Technical Manual

Page: 354 / 804

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 180 , 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.
Y
A
kV
j
Fdtot
= ≈ ⋅
10
15 3
1 15 . mS
GUID-3631BAB9-7D65-4591-A3D6-834687D0E03C V2 EN
(Equation 25)
A parallel resistor current of 15 A can be converted into admittance.
G
A
kV
cc
= ≈
15
15 3
1 73 . mS
GUID-4B7A18DE-68CB-42B2-BF02-115F0ECC03D9 V2 EN
(Equation 26)
According to Equation 13 , during an outside fault EFPADM measures the following
admittance:
Yo Y j
Fdtot
= − ≈ − ⋅
1 15 . mS
GUID-AD02E209-1740-4930-8E28-AB85637CEF0D V2 EN
(Equation 27)
According to Equation 16 , during an inside fault EFPADM measures the admittance
after the connection of the parallel resistor:
Yo Y Y j B
Bgtot CC
= + ≈ + ⋅
( )
1 73 . mS
GUID-28AF4976-1872-48A1-ACC7-7CC3B51CD9D8 V2 EN
(Equation 28)
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"
Section 4
1MRS758755 A
Protection functions
348 REC615 and RER615
Technical Manual