ABB RELION REX640, Техническое руководство, Страница: 583 / 1960

systems. It can be applied for the earth-fault protection of overhead lines
and underground cables, regardless of actual earth-fault type (continuous or
intermittent) or fault resistance value (low or high ohmic). Operation time of IFPTOC
can either be definite time or inverse time. In case of inverse time operation,
operation time becomes automatically adapted to estimated single-phase earth-
fault current or touch-voltage magnitude.
Traditional directional, sensitive earth-fault protection functions applied in high-
impedance earthed networks are typically based on residual quantities (U
o
, I
o
).
In high-impedance earthed networks residual current does not accurately match
the fault current flowing at fault location. This is since the capacitive current
contribution of the faulted feeder itself is not measurable using residual current.
The higher is the capacitive current contribution of the faulted feeder, the greater
is the mismatch of true fault current and residual current (I
o
). This is the root
reason, why operate time cannot be based on measured magnitude of current
(I
o
). Therefore, in traditional directional sensitive earth-fault protection functions
operate time must be pre-defined and it is thus based on
assumed magnitude of
fault current.
Actual fault current magnitude depends on e.g. the prevailing detuning
degree of the network during an earth fault, which depends on the actual values of
network shunt capacitance and coil inductance. In modern networks with increased
degree of cabling, switching operations may result into large variations of network
capacitance. On the other hand, typical practical arrangement for adjustment of
coil inductance involves mechanical movement of air gap in the iron core, which is
a time-consuming process. In case there is great mismatch between the assumed
and true fault current magnitude, the fulfillment of electrical safety regulations
is questionable. It should be noted that the time constant of coil tuning is much
longer than requirements for protection operate time. In modern networks, high
temporary detuning conditions are possible due to increase of use of underground
cables, which increase feeder total phase-to-earth capacitance enormously.
However, operation of IFPTOC is not based on traditional residual quantities (U
o
, I
o
),
but on accurate estimation of earth-fault (EF) current flowing at the fault location.
Estimation of earth-fault current is done in real-time utilizing changes in phase
currents measured at the beginning of the feeder due to a single-phase earth fault.
Thanks to its novel operation principle, the method has several advantages over
the traditional state-of-art earth-fault protection methods such as the wattmetric
method:
• The method enables automatic, real-time adaptation of protection operation
speed according to the prevailing (estimated) single-phase earth-fault current
magnitude. The estimated earth-fault current magnitude can be converted into
corresponding touch voltage or earth potential rise (EPR) value, which enables
direct compliance of protection operation speed according to standard EN50522.
As another option, IFPTOC enables protection operation speed according to
standard IEEE80. This operation mode can also be used when maximum allowed
earth potential rise vs. operation speed follows a relationship such as 750V/
√
t[sec].
• The harmonic content of fault current can be included into fault current
magnitude estimate, which further enhances the accuracy and practicality of the
novel protection method.
• IFPTOC enables significant improvement on safety and overall dependability
of the protection schemes used today in compensated networks. IFPTOC
provides unique advantages, for example during the following practical network
conditions:
- On feeders with high capacitive earth-fault current contribution (e.g. long
cable feeders), where traditional earth-fault protection is challenged by the
1MRS759142 F Protection functions
REX640
Technical Manual
583