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Instruction manual
–
AQ G3x7 Generator protection IED
111 (211
)
The impedance calculation methods
The short explanation of the internal logic for the impedance calculation is as follows:
Calculation method Calc(A):
If the CURRENT_OK status signal is false, the current is very small, therefore no fault is
possible. In this case, the impedance is set to extreme high values and no further calculation
is performed:
R=1000000, X=1000000.
The subsequent decisions are performed if the current is sufficient for the calculation.
Calculation method Calc(B):
If the CURRENT_OK status signal is true and the VOLT_OK_HIGH status signal is true as
well, then the current is suitable for calculation and the voltage is sufficient for the
directionality decision. In this case, normal impedance calculation is performed based on
the sampled currents and voltages. (The calculation method -
the function ”f”
- is explained
later.)
R, X=f(u, i)
If the CURRENT_OK status signal is true but the VOLT_OK_HIGH status signal is false or
there are voltage swings, the directionality decision cannot be performed based on the
available voltage signals temporarily. In this case, if the voltage is above a minimal level (in
the range of possible capacitive voltage transformer swings), then the VOLT_OK_LOW
status is “true”, the magnitude of R and X is calculated based on the actual currents and
voltages but the direction of the fault (the +/- sign of R and X) must be decided based on
the voltage value stored in the memory 80 ms earlier. (The high voltage level setting assures
that during the secondary swings of the voltage transformers, no distorted signals are
applied for the decision). This procedure is possible only if there are stored values in the
memory for 80 ms and these values were sampled during a healthy period.
R, X=f(u, i) direction = f(Umem, i) /in the first 35 ms/
After 35 ms (when the secondary swings of the voltage transformers decayed), the
directional decision returns to the measured voltage signal again:
R, X=f(u, i) direction = f(u, i) /after 35 ms/