ABB RELION RET670, Applications Manual

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• 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.
9.16.2.2 Generator continuous unbalance current capability
GUID-7C2DBEA9-4423-47DD-90FD-88F4AAD2E9A9 v3
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
2
and rotor heating criteria
2
2
I t
.
Typical short-time capability (referred to as unbalanced fault capability) expressed in
terms of rotor heating criterion
2
2
I t K
=
is shown below in Table 46 .
Table 46: ANSI requirements for unbalanced faults on synchronous machines
Types of Synchronous Machine
Permissible
[ ]
2
2
I t K s
=
Salient pole generator 40
Synchronous condenser 30
Cylindrical rotor generators: Indirectly cooled 30
Directly cooled (0 – 800 MVA) 10
Directly cooled (801 – 1600
MVA)
See Figure 292
Fig 292 shows a graphical representation of the relationship between generator
2
2
I t
capability and generator MVA rating for directly cooled (conductor cooled) generators.
For example, a 500 MVA generator would have K = 10 seconds and a 1600 MVA
generator would have K = 5 seconds. Unbalanced short-time negative sequence current
I
2
is expressed in per unit of rated generator current and time t in seconds.
Section 9
1MRK 504 163-UUS A
Current protection
602 Transformer protection RET670 2.2 ANSI
Application manual