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1
1.1
1.2
1.3
64
Instantaneous non-directional overcurrent
protection (IOC)
Instantaneous non-directional overcurrent
protection (IOC)
Application
Different system conditions, such as source impedance and the position of the faults on long
transmission lines influence the fault currents to a great extent. An instantaneous phase overcur-
rent protection with short operate time and low transient overreach of the measuring elements
can be used to clear close-in faults on long power lines, where short fault clearing time is ex-
tremely important to maintain system stability.
The instantaneous residual overcurrent protection can be used in a number of applications. Be-
low some examples of applications are given.
•
Fast back-up earth fault protection for faults close to the line end.
•
Enables fast fault clearance for close in earth faults even if the distance protection
or the directional residual current protection is blocked from the fuse supervision
function
Functionality
The current measuring element continuously measures the current in all three phases and com-
pares it to the set operate value IP>>. A filter ensures immunity to disturbances and dc compo-
nents and minimizes the transient overreach. If any phase current is above the set value IP>>,
the phase overcurrent trip signal TRP is activated. Separate trip signal for the actual phase(s) is
also activated. The input signal BLOCK blocks all functions in the current function block.
The current measuring element continuously measures the residual current and compares it to
the set operate value IN>>. A filter ensures immunity to disturbances and dc components and
minimizes the transient overreach. If the residual current is above the set value IN>>, the resid-
ual overcurrent trip signal TRN is activated. The general trip signal TRIP is activated as well.
The input signal BLOCK blocks the complete function.
Function block
IOC--
IOC
BLOCK
TRIP
TRP
TRL1
TRL2
TRL3
TRN
xx00000201.vsd
Figure 19:
IOC function block phase + N with 1, 2, 3 phase trip
Chapter 5
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