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2 Functions
2.7
Definite-Time Overcurrent Protection (I>>, ANSI 50, 51, 67) with
Direction Detection
The overcurrent protection is used as backup protection for the short-circuit protection
of the protected object. It also provides backup protection for downstream network
faults which may be not promptly disconnected thus endangering the protected object.
In order to ensure that pick-up always occurs even with internal faults, the protection
- for generators - is usually connected to the current transformer set in the neutral
leads of the machine. If this is not the case for an individual power system, the I>>
stage can be combined with a short-circuit direction acquisition and switch off a gen-
erator short circuit by undelayed tripping without comprising selectivity.
Initially the currents are numerically filtered so that only the fundamental frequency
currents are used for the measurement. This makes the measurement insensitive to
transient conditions at the inception of a short-circuit and to asymmetrical short-circuit
currents (d.c. component).
2.7.1
Function Description
Each phase current is compared individually with the I>> common pick-up value and
I>> Stage
signaled on overshoot. A trip signal is transmitted to the matrix as soon as the corre-
sponding T I>> time delays have expired. The dropout value is ± 95 % below the
pick-up value.
Direction Detection
The I>> stage is equipped with a (disconnectable) direction element permitting a trip-
ping only for faults in backward (i.e. machine) direction.
For this reason, this stage can be used particularly in applications where no current
transformers exist in the generator starpoint and undelayed tripping is nevertheless re-
quired on generator faults.
Figure 2-5
The direction is detected phase-selectively by means of a cross-polarized voltage.
The phase-to-phase voltage normally perpendicular to the fault current vector is used
as unfaulted voltage (Figure 2-6). This is considered during the calculation of the di-
rection vector in the clockwise rotating phase sequence by a +90° rotation, and in the
anti-clockwise rotating phase by a -90° rotation. For phase-to-phase faults, the posi-
tion of the directional limit lines may change in relation to the collapse of the fault volt-
age.
46
Selectivity via Short-Circuit Direction Detection
7UM61 Manual
C53000-G1176-C127-3
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