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2 Functions
2.8
Inverse-Time Overcurrent Protection (ANSI 51V)
The overcurrent time protection replicates the short-circuit protection for small or low-
voltage machines. For larger machines it is used as back-up protection for the
machine short-circuit protection (differential protection and/or impedance protection).
It provides back-up protection for network faults which may be not promptly discon-
nected thus endangering the machine.
In generators where the excitation voltage is taken from the machine terminals, the
short-circuit current subsides quickly in the event of adjacent faults (i.e. in the gener-
ator or unit transformer region) due to the absence of excitation voltage. Within a few
seconds it sinks below the pick-up value of the overcurrent time protection. In order to
avoid a dropout of the pickup, the positive-sequence component is monitored addition-
ally. This component can influence the overcurrent detection in accordance with two
different methods. Undervoltage influencing can be disabled.
The protective function operates, depending on the ordering variant, with an inverse
current-tripping characteristic according to the IEC or ANSI standards. The character-
istics and associated formulas are given in the Technical Data. During configuration of
the inverse time curves, the independent stages I>> and I> are enabled (see Section
2.6).
2.8.1
Functional Description
Each phase current is compared individually with the common Ip setting value. If the
Pickup and Trip-
ping
current exceeds 1.1 times the setting value, the stage picks up and is signaled phase-
related. For pickup the rms values of the fundamental harmonic are used. During the
pickup of an Ip stage, the tripping time is calculated from the flowing fault current using
an integrating measuring procedure, depending on the selected tripping characteris-
tic. After the end of this period a trip command is transmitted.
Dropout
The dropout of a picked up stage is performed as soon as the value falls below ap-
proximately 95 % of the pickup value (i.e. 0.95 to 1.1 = 1.045 to setting value). A
renewed pickup will the delay timer to start anew.
Undervoltage Con-
The inverse overcurrent time protection is provided with a undervoltage detection that
sideration
can be disabled. This function can influence overcurrent detection in two different
ways:
• Voltage controlled: If the value falls below a settable voltage threshold, an over-
• voltage restraint: The pickup threshold of the overcurrent stage depends on the
52
current stage with a lower pick-up value is enabled.
voltage level. A lower voltage reduces the current pickup value (see the following
figure). A linear, directly proportional dependency is used in the zone between
U/U
= 1.00 and 0.25. Consequently, the following applies:
Nom
7UM61 Manual
C53000-G1176-C127-3
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