Earth-fault current and touch voltage protection
Based on the practical experience from real earth faults, some faults have re-
striking/intermittent characteristics, where the voltage and current waveforms
generated by earth fault are rich with harmonics and non-sinusoidal content. In
such fault type the operation of IFPTOC can alternatively be based on the counted
number of transients instead of estimated fault current or touch voltage. Dedicated
functionality to allow this type of operation is described in
Chapter 4.2.10.5.6
Intermittent earth-fault protection
.
Principle based on earth fault current estimate
The operation of IFPTOC based on earth-fault current estimate is described next.
In case
Operation principle
= “EF current based”, then operation is based on
the estimated effective earth-fault current magnitude taking account the effect
of
Reduction factor
. The harmonic content of fault current can be included into
estimate with setting
Enable harmonics
= “Enable”.
Operation time can be either definite time or inverse time, selected with setting
Operating curve type
= “Definite time”, “Inverse time EN50522” or “Inverse time
IEEE80”.
When
Operating curve type
= “Definite time” is selected, then
• Operate time is according to definite time (DT) operation
• START and ST_EF outputs are activated when estimated earth-fault current
magnitude (considering the effect of settings
Reduction factor
and
Enable
harmonics
) exceeds setting
EF current Str Val
and earth fault is detected, and
earth-fault current estimate is validated. Note that
Ena RF Compensation
=
“Enable” does not affect this start condition. It is important to set setting
EF
current Str Val
according to required protection sensitivity.
• OPERATE and OP_EF outputs are activated after operation timer has elapsed,
defined with setting
DT stage Op time
.
• Reset timer is started if any of the above conditions for START is not anymore
valid. START and ST_EF outputs stay activated until the reset timer expires
(setting
Reset delay time
). After OPERATE output activation, START, ST_EF,
OPERATE and OP_EF outputs are reset immediately i.e. reset delay time is no
longer valid.
• During re-striking/intermittent earth faults, in case operation should be
based preferably on counted number of transients, then setting
Reset
delay time
should be set to low value, say 20-40 ms, thus definite time
operation is ineffective during re-striking/intermittent earth faults as DT-timer
becomes reset between re-strikes. Dedicated intermittent earth-fault protection
functionality based on counted number of transients is described in
Chapter
4.2.10.5.6 Intermittent earth-fault protection
.
• Applicable settings are:
Reduction factor, EF current Str Val, DT stage Op time
and
Reset delay time
.
• Note that setting
CB Delay Comp
(circuit breaker delay compensation) is not
applicable in definite time operation, but it must be taken into account in setting
DT stage Op time
.
Minimum practical value for setting
EF current Str Val
is 0.5% from CT
primary current. For example, in case CT primary current is 200 A, then
minimum practical value for setting
EF current Str Val
is 0.5%*200A =
1 A. For CT with 400 A primary current, then minimum practical value
for setting
EF current Str Val
is 0.5%*400A = 2 A. The lower setting
EF
current Str Val
is used, the higher sensitivity in terms of fault resistance
Protection functions 1MRS759142 F
598 REX640
Technical Manual
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