Negative Sequence Protection 46; Definite Time Element 46-1, 46-2 - Siemens siprotec SJ62 User Manual

2.7

Negative Sequence Protection 46

Applications
Prerequisites
2.7.1

Definite Time element 46-1, 46-2

SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
Negative sequence protection detects unbalanced loads on the system.
• The application of negative sequence protection to motors has a special signifi-
cance. Unbalanced loads create counter-rotating fields in three-phase induction
motors, which act on the rotor at double frequency. Eddy currents are induced on
the rotor surface, which causes local overheating in rotor end zones and the slot
wedges. This especially goes for motors which are tripped via vacuum contactors
with fuses connected in series. With single phasing due to operation of a fuse, the
motor only generates small and pulsing torques such that it soon is thermally
strained assuming that the torque required by the machine remains unchanged. In
addition, the unbalanced supply voltage introduces the risk of thermal overload.
Due to the small negative sequence reactance even small voltage asymmetries
lead to large negative sequence currents.
• In addition, this protection function may be used to detect interruptions, faults, and
polarity problems with current transformers.
• It is also useful in detecting 1 pole and 2 pole faults with fault current lower than the
maximum load current.
In order to prevent pickup chattering, the negative sequence protection becomes only
active when one phase current becomes larger than 0.1 x I
are smaller than 4 x I
Nom
The definite time characteristic consists of two elements. As soon as the first settable
threshold 46-1 PICKUP is reached, a pickup message is output and time element
46-1 DELAY is started. When the second element 46-2 PICKUP is started, another
message is output and time element 46-2 DELAY is initiated. Once either time delay
elapses, a trip signal is initiated.
Figure 2-47 Definite time characteristic for negative sequence protection
.
2.7 Negative Sequence Protection 46
and all phase currents
Nom
146