ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-68
E. Setting instructions
Settings:
Current pick-up I-Setting
Delay Delay
Minimum frequency f-min
Over or undercurrent MaxMin
The instantaneous overcurrent function is a high-speed protec-
tion which operates in a wide frequency range. It is intended
primarily for two applications.
A protection measuring peak value is necessary for protecting
units, for which the influence of DC component and harmonics
may not be neglected. This is especially the case where rectifi-
ers with semiconductors are involved.
The measuring principle of the function is relatively insensitive to
frequency and operates in a range extending from 4 to 120 % of
rated frequency. It is therefore able to protect units with syn-
chronous starting equipment during the starting sequence before
reaching system frequency (e.g. gas turbine sets with solid-state
starters).
The function detects when the instantaneous value of the input
current exceeds the peak value corresponding to the setting. For
example, for a setting of 10 I
N
, it will pick up when the input cur-
rent exceeds 10
2 I
N
= 14.14 I
N
(see Fig. 3.5.4.1). A fault cur-
rent of 6 x 1.8
2 I
N
= 15.27 I
N
could reach this level as a conse-
quence of a DC component.
The minimum frequency must be entered for every application,
because it determines the reset time. A low minimum frequency
means a long reset delay and since a good protection is ex-
pected to have a quick response, the reset time should be as
short as possible, i.e. the minimum frequency setting should not
be lower than absolutely necessary.
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