ABB RELION REX640 - Page 587

Figure 336: Example of selection of setting Voltage start value in order to maximize
the earth-fault detection sensitivity of IFPTOC based on the resonance curve
calculated by the coil controller
To avoid unselective start or operation of IFPTOC,
Voltage start value
must always be set to a value which exceeds the maximum healthy-state
residual voltage value, taking into consideration of possible network
topology changes (variation in unbalance), compensation coil and parallel
resistor switching status (variation in damping) and compensation
degree variations (variation in detuning).
Activation of EF_IND output is determined to be the time instant when the
earth fault is detected. This information is needed as the IFPTOC utilizes “delta”
calculation i.e. changes in phase currents due to earth fault. For example, the
change of phase A current phasor due to earth fault can be written as:
(Equation 102)
Where:
t
FLT
is the time during the detected earth fault
t
PRE_FLT
is the time prior to detected earth fault occurrence and it represents
time during pre-fault or healthy-state conditions. This pre-fault operation point is
determined based on user setting Revert time as follows:
(Equation 103)
1MRS759142 F
Protection functions
REX640
Technical Manual
587