Weak infeed

[جعفر اغائي]

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[elhusinyelhabyan]

Weak infeed or weak source scenario occurs only under fault condition. But this system
configuration may exist under condition of normal system operations or some kind of
outage operations. Generally there are two kinds of weak infeed scenario, conditionally
and unconditionally, depend on system configuration and types of power sources.
The truth of weak infeed is fault impedance, the sum of source impedance and short
circuit impedance, is higher and power sources can’t provide reliable fault current for
relay current related elements detecting the fault. An interesting issue is fault impedance
is a variable function of system operation, and especially fault location within the
protection zone. Under a given system condition fault impedance is nonlinearly
proportional to fault location. For close in fault it is the minimum and for remote end
fault it is the maximum.

Conditional weak infeed occurs in complex system configuration and is dependent on
fault location. Protection for conditional weak infeed is included in line protection
schemes that apply manufacturer’s typical logics generally.
Unconditional weak infeed occurs mainly with IPP (Independent Power Producers)
connection to the system. For the innate features of IPP sources, its equivalent source
impedances may limit to provide fault current to some extent for external faults in grid.
Protection for unconditional weak infeed is not included in line’s protection schemes

generally, but practiced as a way of interconnection protection. The protection should
include two parts, the main protection from utilities line’s ends, and independent IPP
local backup protection.