5.3.20 Voltage-restrained overcurrent protection (Iv>; 51V)
Short-circuits that occur close to the generator decrease the fault current which in turn inhibits the
operation of a high-set overcurrent stage. The decreasing voltage caused by these faults can be used
to decrease the current pick-up level and thus to improve sensitivity. This voltage-restrained
overcurrent protection function can be used as an alternative for the underimpedance function for
more sensitive short-circuit detection in generator protection applications.
When there is a short-circuit near the generator, the voltage decreases which in this function's case
decreases the overcurrent's pick-up level according to set parameters. The function can work as
voltage-controlled overcurrent protection or as voltage-restrained overcurrent protection. When the set
parameter value Ux2 is greater than Ux1, the protection function works as voltage-restrained
overcurrent protection. In this case the overcurrent pick-up value increases as the voltage increases
between the set values Ux1 and Ux2. The protection function uses positive sequence voltage to define
the pick-up level at that moment.
When the set value of Ux1 is equal to Ux2, the function works as voltage-controlled overcurrent
protection. Now the overcurrent protection doesn’t operate until a fault reduces the voltage below a set
value, usually about 80 % of the normal. A fixed pick-up level of voltage-controlled overcurrent
protection is easier to coordinate with other relays. However, the voltage-restrained overcurrent
protection function is less prone to making unwanted operations on motor starting currents and system
swings.
Figure. 5.3.20 - 170. Pick-up levels in the two modes.
Just like the other overcurrent protection functions, this function can be set to definite time (DT) or
inverse definite minimum time (IDMT) delay. However, if IDMT is selected for this function, the time
delay depends on the ratio between the measured current and the current pick-up level at
that moment. This means that the operation time can also shorten as a result of the reduced voltage.
A
AQ
Q-G215
-G215
Instruction manual
Version: 2.04
247
Содержание AQ-G215
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