ABB RET670 - Setting Guidelines

Table 121: 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
• Single phase railroad load
• Unbalanced system faults such as
• Line to ground faults
• Double line to ground faults
• Line to line faults
• Open conductors, includes
• Broken line conductors
• Malfunction of one pole of a circuit breaker
.
3.7.14.2 Setting guidelines
When inverse time overcurrent characteristic is selected, the operate
time of the stage will be the sum of the inverse time delay and the set
1MRK504116-UUS C Section 3
IED application
505
Application manual