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2.10
Inverse-Time Overcurrent Protection (ANSI 51V)
The inverse-time overcurrent protection protects extra-low voltage and low-voltage machines against short cir-
cuits. 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 that can not be
cleared immediately and thus endanger the machine.
The 7UM62 relay allows to choose between the input transformers of side 1 and side 2 for allocation of the
inverse-time overcurrent protection function. This choice is made during configuration (see Section 2.4).
In generators where the excitation voltage is taken from the machine terminals, the short-circuit current sub-
sides quickly in the event of adjacent faults (i.e. in the generator 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 additionally. This com-
ponent can influence the overcurrent detection in accordance with two different methods. The undervoltage in-
fluencing can be switched off.
The protective function operates, depending on the ordering variant, with an inverse current-tripping character-
istic according to the IEC or ANSI standards. The characteristic curves and the corresponding formulas are
represented in Technical Data. If one of the inverse characteristics (IEC or ANSI) is configured, the definite-
time stages I>> and I> can be additionally effective (see Section 2.8).
2.10.1
Functional Description
Pickup and Tripping
Each phase current is compared individually with the common Ip setting value. If a current exceeds 1.1 times
the set value, the stage picks up and is signalled on a per phase basis. The r.m.s. values of the fundamental
component are used for the pickup. During the pickup of an Ip stage, the tripping time is calculated from the
flowing fault current by means of an integrating measuring procedure, depending on the selected tripping char-
acteristic. After the expiration 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 approximately 95 % of the
pickup value (i.e. 0.95 to 1.1 = 1.045 to setting value). The timer will start again for all new pickups.
Undervoltage Detection
The inverse overcurrent time protection is provided with a undervoltage detection that 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 overcurrent stage is enabled.
• Voltage restraint:The pickup threshold of the overcurrent stage depends on the voltage level. A lower
voltage reduces the current pickup value (see Figure 2-18). A linear, directly proportional dependency is re-
alized in the zone between U/U
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
= 1.00 to 0.25. Consequently, the following rule applies:
Nom
2.10 Inverse-Time Overcurrent Protection (ANSI 51V)
Functions
75
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