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Two step definite and inverse time
delayed phase overcurrent protection
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The non-directional two-step time delayed overcurrent protection is used as short-cir-
cuit protection in single-phase and two-phase systems. It is intended to be used either
as primary protection or back-up protection for the impedance measuring functions.
In radial networks it is often sufficient to use phase overcurrent relays as short circuit
protection for lines, transformers and other equipment. The current time characteristic
should be chosen according to common practice in the network. It is strongly recom-
mended to use the same current time characteristic for all overcurrent relays in the net-
work. This includes overcurrent protection for transformers and other equipment.
There is a possibility to use phase overcurrent protection in meshed systems as short cir-
cuit protection. It must however be realized that the setting of a phase overcurrent pro-
tection system in meshed networks, can be very complicated and a large number of fault
current calculations are needed. There are situations where there is no possibility to
have selectivity with a protection system based on overcurrent relays in a meshed sys-
tem.
The measuring function contains one current measuring element for each phase, each
of them with a low set and a high set measuring step. The low set step can have either
definite time or inverse time characteristic. The characteristics available are extremely
inverse, very inverse, normal inverse or RI inverse. The high set step has definite time
delay.
The settings are common for all phases but both the low and high set step can be set On/
Off individually and also have individual inputs for blocking.
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The current measuring elements within one of the integrated digital signal processors
continuously measure the current in the current circuits and compares it with the set op-
erate value. The current signals are filtered using a Fourier recursive filter and a sepa-
rate trip counter prevents high overreaching.
The simplified logic diagram of the time delayed phase overcurrent function is shown
in figure 44.
The function is disabled (blocked) if:
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