Application Guide - NP900 Series
290 (504
)
A996A
3.4.3
U
NDER IMPEDANCE
Z<
(21G)
Underimpedance protection is an alternative for voltage restrained overcurrent protection
(also included in the NPG915 model) which can be used to detect short circuit faults near
the generator even when the short circuit current is small.
Figure 3.4.3-142 Simplified function block diagram of the UIM function.
3.4.3.1
M
EASURED INPUT VALUES
Sufficient current and voltage measurements are required for the impedance
measurement to work properly. By user selection the monitored magnitude can be P-E
impedance loops or positive sequence impedance. For the pre-fault data registering -20
ms averaged value of the selected magnitude is used.
Table 3.4.3.1-190 Measurement magnitudes used by the UIM function.
Signal
Description
Time base
Z1
Peak-to-peak measurement of phase L1/A current
5 ms
Z2
Peak-to-peak measurement of phase L2/B current
5 ms
Z3
Peak-to-peak measurement of phase L3/C current
5 ms
3.4.3.2
P
ICK
-
UP CHARACTERISTICS
Pick-up level of the UIM function is controlled by setting parameters Zset(pri)<. Pick-up
level defines the minimum allowed measured impedance before action from the function.
The function constantly calculates the ratio in between of the impedance pick-up level and
