Setting EF Start Cur must be set above the highest threefold negative-
sequence current value measured during the healthy state.
In order to provide reliable and selective earth-fault detection, the previously
described earth-fault detection conditions,
Equation 18 and Equation 19, must be
fulfilled for the minimum duration as defined with setting Start Delay Time. The pick-
up timer is, however, overridden, if simultaneously the residual current amplitude and
the estimated earth-fault current amplitude exceeds setting Inst Start Cur:
I
o
AND I
EF
>Inst Start Cur
Setting Inst Start Cur should be set to high value, which indicates
single-phase earth fault with abnormal high earth-fault current value.
Such a condition could be due to, for example, malfunction of the Arc
suppression Coil itself or the coil tuner. Setting value should be based
on magnitude of the uncompensated earth fault current of the network.
Setting Inst Start Cur is set lower value than the uncompensated earth
fault current of the network with sufficient margin.
After an earth fault is detected by the General fault criterion module, calculation of
‘delta’ quantities for residual current and earth-fault current is conducted. Calculation
of change in the amplitude of residual current (ΔI
o
) and in the earth-fault current (ΔI
F
)
due to earth fault is done by comparing the present value of residual current phasor
(I
o
) and earth fault current phasor (I
F
) to value Revert Time earlier and calculating its
amplitude:
ΔI
o
= abs(I
o
(t
FLT
) - I
o
(t
PRE_FLT
))
ΔI
EF
= abs(
I
EF
(t
FLT
) - I
EF
(t
PRE_FLT
))
where
t
FLT
is the time during the earth fault
t
PRE_FLT
is the time prior to the earth fault and it represents pre-fault or healthy-state
conditions. This pre-fault operation point is determined based on user setting Revert
Time as follows:
t
PRE_FLT
= t
FLT
(1) – Revert Time
where
t
FLT
(1) equals the first time instance, when the earth fault is internally detected by
FPIPTOC, based on
Equation 18 and Equation 19. Pre-fault time moment is at least
Revert Time before earth fault occurrence moment.
In case of high(er) ohmic earth-fault, due to slow increase of Io and I
F
magnitude, it may take several hundreds of milliseconds until setting
thresholds are exceeded and an earth fault is detected after actual
Section 6 1MRS757488 H
Commissioning
152 RIO600
Installation and Commissioning Manual
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