ABB RET670 Applications Manual page 509

1MRK504116-UUS C
Application manual
A separate output is available as an alarm feature to warn the operator of a potentially
dangerous situation.
Features
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 for step 1:
• Definite time delay
• Inverse time delay
•
The inverse time overcurrent characteristic matches
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.
• Maximum trip time delay for inverse time characteristic, freely settable.
• Inverse reset characteristic which approximates generator rotor cooling rates and
provides reduced operate time if an unbalance reoccurs before the protection resets.
• Service value that is, measured negative sequence current value, in primary
Amperes, is available through the local HMI.
Generator continuous unbalance current capability
During unbalanced loading, negative sequence current flows in the stator winding.
Negative sequence current in the stator winding will induce double frequency current
in the rotor surface and cause heating in almost all parts of the generator rotor.
When the negative sequence current increases beyond the generator's continuous
unbalance current capability, the rotor temperature will increase. If the generator is not
tripped, a rotor failure may occur. Therefore, industry standards has been established
that determine generator continuous and short-time unbalanced current capabilities in
terms of negative sequence current I
Typical short-time capability (referred to as unbalanced fault capability) expressed in
terms of rotor heating criterion
and rotor heating criteria
2
2
=
I t K
is shown below in Table 120.
2
Section 3
IED application
2
=
I t K
capability curve of
2
2
I t
.
2
503