Negative Sequence Protection 46
Negative sequence protection detects unbalanced loads on the system.
Applications
•
The application of unbalanced load protection to motors has a special significance. Unbalanced loads
create counter-rotating fields in three-phase induction motors, which act on the rotor at double
frequency. Eddy currents are induced at the rotor surface, and local overheating in rotor end zones and
the slot wedge begins to take place. This especially goes for motors which are tripped via vacuum contac-
tors with fuses connected in series. With single-phasing by fuse pickup the motor only generates small
and pulsing moments such that it soon gets strained thermally assuming, however, that the driven
machine requires the same amount of moments. In addition, with unbalanced supply voltage it is endan-
gered by thermal overload. Due to the small negative sequence reactance even small voltage asymme-
tries lead to negative sequence currents.
•
In addition, this protection function may be used to detect interruptions, short circuits and polarity prob-
lems with current transformers.
•
It is also useful in detecting single-phase and two-phase faults with fault currents lower than the
maximum load currents.
Prerequisites
The unbalanced load protection becomes effective when:
at least one phase current is greater than 0.05 x Ι
Nom
and
all phase currents are smaller than 10 x Ι
Nom
.
Definite Time characteristic
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.
[unabhaeng-ausl_charakt-des-schieflastschutzes-260602-kn, 1, en_US]
Figure 2-52 Definite time characteristic for negative sequence protection
2.7
2.7.1
Functions
2.7 Negative Sequence Protection 46
SIPROTEC 4, 7SJ62/64, Manual 137
C53000-G1140-C207-8, Edition 08.2016
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