ABB REO 517 Applications Manual page 90




80
High speed and instantaneous phase
overcurrent protection (HSOC, IOC)
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The protection is intended for single-phase and two-phase systems with a rated frequen-
cy of 16 2/3, 50 or 60 Hz.
The high speed and instantaneous overcurrent protection functions are non-directional
and normally serves as a local backup function to the distance protection.
The overcurrent protection has two operating modes:
• HSOC that measures on sample level and is faster than the IOC, but has a larger
transient overreach.
• IOC that measures on a filtered signal and has a limited overreach
The fault current on long lines depends mostly on the fault position and decreases with
the distance from the generation point. Faults very close to the generation (and relay)
point, for which very high fault currents are characteristic, should be cleared as quickly
as possible.
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The current measuring element in one of the integrated signal processors continuously
measures the phase currents (IL1 and IL2) in two-phase systems and current IL1 in sin-
gle phase systems. An elliptical filter is used to filter the current signals before the ef-
fective value is calculated for the IOC function. The calculated effective value is
compared with a set operate value I>> and the output signal IOC-TRIP is activated in-
stantaneously if the measured current exceeds the operate value. A separate counter is
used as protection against large overreaching.
For the other, faster function, HSOC, a direct sample value is used, but a separate
counter protects the function from operation on individual current peaks. The signal is
compared with a set operate value, I>>> and the output signal HSOC-TRIP is activated
instantaneously if the measured current exceeds the operate value.
The simplified logic diagram of the instantaneous phase overcurrent function is shown
in figure 41.
The IOC function is disabled if:
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