ABB 615 series Technical Manual page 403

1MAC059074-MB A
615 series ANSI
Technical Manual
According to Equation 26, during an outside fault 21YN measures the following
admittance:
Yo Y ≈ − ⋅1 15 j . mS
= −
Fdtot
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According to Equation 29, during an inside fault 21YN measures the admittance after the
connection of the parallel resistor:
Yo Y Y 1 73 .
(
= + ≈
Bgtot CC
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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"
Directional mode = "Non-directional"
The admittance characteristic is set to cover the total admittance of the protected feeder
with a proper margin, see
adaptation of protection settings to different feeder and network configurations.
Conductance forward
This setting should be set based on the parallel resistor value of the coil. It must be set to
a lower value than the conductance of the parallel resistor, in order to enable dependable
operation. The selected value should move the boundary line from origin to include some
margin for the admittance operation point due to CT/VT-errors, when fault is located
outside the feeder.
Conductance forward: 15 A/(15 kV/sqrt(3)) * 0.2 = +0.35 mS corresponding to 3.0 A (at
15 kV). The selected value provides margin considering also the effect of CT/VT-errors
in case of outside faults.
In case of smaller rated value of the parallel resistor, for example, 5 A (at 15 kV), the
recommended security margin should be larger, for example 0.7, so that sufficient margin
for CT/VT-errors can be achieved.
Susceptance forward
By default, this setting should be based on the minimum trip current of 1 A.
Susceptance forward: 1 A/(15 kV/sqrt(3)) = +0.1 mS
j B mS
)
+ ⋅
Figure 191. Different setting groups can be used to allow
Section 4
Protection functions
(Equation 40)
(Equation 41)
397