55169 MiCom

(OP) 5-5/12

MiCOM P115

2.    OPERATION OF INDMDUAL PROTECTION FUNCTIONS

The following sections detail the individual protection functions.

2.1    Overcurrent protection

The overcurrent protection included in the P115 relays provides three-stage non-directional three-phase overcurrent protection with independent time delay characteristics. Ali overcurrent settings apply to all three phases but are independent for each of the three stages.

The first (I>) and second (I») stage of overcurrent protection has time-delayed characteristics which are selectable between inverse definite minimum time (IDMT), or definite time (DMT). The third (I>») stage has a definite time characteristic only.

Various methods are available to achieve correct relay co-ordination on a system; by means of time alone, current alone or a combination of both time and current. Grading by means of current is only possible where there is an appreciable difference in fault level between the two relay locations. Grading by time is used by some Utilities but can often lead to excessive fault clearance times at or near source substations where the fault level is highest. For these reasons the most commonly applied characteristic in coordinating overcurrent relays is the IDMT type.

The inverse time delayed characteristics indicated above comply with the following formulae:

IEC curves: t = TMS--——;

M^a-\

IEEE/ANSI curves:    t = TD{—-— + /.);

M*-\

where:
t	Operating time in [s]
P	Constant
M = — Is
I	Measured current in (A]
TMS =	Time setting [s]
Is =	Current threshold setting
a =	Constant
L	ANSI/IEEE Constant