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(iv) Calculation error
User should note that the calculation error could be larger when a setting value is far from
actual line impedance. For example, if abnormal settings
1
are done, the calculation error will
be appeared notably.
The calculation error can be with in ± 2.5 km when the following conditions are applied:
⚫ A fault is assumed to arise up to 100 km distance.
⚫ The fault current is larger than double of the rated current
2
(
).
⚫ The reactance
3
per unit length (
) is longer than a value (
).
In other words, this calculation error can be with in ±2.5% when the following conditions
are applied:
⚫ The line length (L) is shorter than 100km.
⚫ The reactance per unit length (
) is larger than a value (
).
⚫ The fault current is larger than double of the rated current
2
(
).
1
Note: Abnormal settings can be exist when values are far from actual impedance data of
transmission line. For example, resistance value is so larger then reactance value,
and so on.
2
Note: Rated current (In) is described separately. (See Chapter
Technical description:
Transformer module
)
3
Note: The term X1 is the reactance component of line impedance.
2.32.3 Selection of calculation results
At the outset, we discuss that the FL calculation consist of two groups: (1) FL calculation with
data at a local-terminal and remote-terminals and (2) FL calculation with data at a local-
terminal only. Hence, either (1) or (2) is selected as a calculation result in accordance with a
fault condition. The FL selects the result of (2) FL calculation with data at a local-terminal
only when the FL finds the following conditions after the calculations:
⚫ The calculation results are less than 20% of the line length (L).
⚫ The result difference between (1) and (2) is within 5% of the line length (L).
⚫ The calculation result is less than 40km.
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