ABB RELION RET670, Applications Manual

Page: 607 / 1016

To provide an effective protection for the generator for external unbalanced conditions,
NS2PTOC (46I2) is able to directly measure the negative sequence current. NS2PTOC
(46I2) also have a time delay characteristic which matches the heating characteristic of
the generator I
2
2
t = K as defined in standard.
where:
I
2
is negative sequence current expressed in per unit of the rated generator current
t is operating time in seconds
K is a constant which depends of the generators size and design
A wide range of I
2
2
t settings is available, which provide the sensitivity and capability
necessary to detect and trip for negative sequence currents down to the continuous
capability of a generator.
A separate output is available as an alarm feature to warn the operator of a potentially
dangerous situation.
9.16.2.1 Features
GUID-9B139200-C5FD-4870-93FD-2EB16B206B25 v4
Negative-sequence time overcurrent protection NS2PTOC (46I2) is designed to
provide a reliable protection for generators of all types and sizes against the effect of
unbalanced system conditions.
The following features are available:
• Two steps, independently adjustable, with separate tripping outputs.
• Sensitive protection, capable of detecting and tripping for negative sequence
currents down to 3% of rated generator current with high accuracy.
• Two time delay characteristics:
•
Definite time delay
• Inverse time delay
•
The inverse time overcurrent characteristic matches
2
2
I t K
=
capability curve of
the generators.
• Wide range of settings for generator capability constant K is provided, from 1 to
99 seconds, as this constant may vary greatly with the type of generator.
• Minimum trip time delay for inverse time characteristic, freely settable. This
setting assures appropriate coordination with, for example, line protections.
1MRK 504 163-UUS A
Section 9
Current protection
Transformer protection RET670 2.2 ANSI 601
Application manual