ABB RELION RET670, Applications Manual

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en08000358.vsd
IEC08000358 V1 EN-US
Figure 292: Short-time unbalanced current capability of direct cooled generators
Continuous I
2
- capability of generators is also covered by the standard. Table 47 below
(from ANSI standard C50.13) contains the suggested capability:
Table 47: Continous I
2
capability
Type of generator Permissible I
2
(in percent of rated
generator current)
Salient Pole: with damper winding 10
without damper winding
5
Cylindrical Rotor
Indirectly cooled 10
Directly cooled
to 960 MVA 8
961 to 1200 MVA 6
1201 to 1500 MVA 5
As it is described in the table above that the continuous negative sequence current
capability of the generator is in range of 5% to 10% of the rated generator current.
During an open conductor or open generator breaker pole condition, the negative
sequence current can be in the range of 10% to 30% of the rated generator current.
Other generator or system protections will not usually detect this condition and the
only protection is the negative sequence overcurrent protection.
Negative sequence currents in a generator may be caused by:
• Unbalanced loads such as
1MRK 504 163-UUS A
Section 9
Current protection
Transformer protection RET670 2.2 ANSI 603
Application manual