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94
Two step definite and inverse time
delayed phase overcurrent protection
(TOC2)
The time-delay characteristic of the low current step function can be set with the setting
Characteristic = x, where x is selected from the following:
•
NI (Normal inverse)
•
VI (Very inverse)
•
EI (Extremely inverse)
•
RI (Inverse time corresponding to relay type RI)
•
Def (Definite time delay)
The different inverse time curves are defined in section ³&DOFXODWLRQV´.
The low current step function is activated by setting Operation Low=On. If an inverse
time characteristic is selected and the current exceeds the set operate value I>Low, the
timer tMinInv is activated and the inverse time circuit is activated in order to start the
calculation of operate time. The operate time is determined by the magnitude of the cur-
rent, selected inverse time-delay characteristic, set characteristic current I>Inv and the
set time multiplicator k. When both the inverse time circuit and the timer tMinInv are
operational, the timer tLow is activated, which after the set delay activates the signal
TOC2-TRLS.
If definite time characteristic is selected and the current exceeds the set operate value
I>Low, the signal TOC2-TRLS is activated after the set time tLow.
Activate the high current step function by setting Operation High=On. The tHigh timer
starts when the ILn current is equal to or higher than the set operate value for I>High.
The signal TOC2-TRHS is activated after the set delay tHigh.
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The parameters for the two step definite and inverse time delayed overcurrent protec-
tion function are set via the local HMI or PST (Parameter Setting Tool). Refer to the
Technical reference manual for setting parameters and path in local HMI.
The phase overcurrent protection can be used in different applications. In most applica-
tions it is required that all short circuits within a protected zone shall be detected and
cleared and the fault clearance shall be selective. As the protection can be used in sev-
eral applications only some examples are discussed.
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The phase overcurrent protection is suitable to use in radial systems without any fault
current infeed from the radial feeders.
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