ABB REG650 Technical Manual page 111

1MRK 502 034-UEN -
Technical Manual
IL1n
External fault: IL1n = - IL1t
IL1t
IEC07000019 V2 EN
Figure 49: External fault
Generator differential protection GENPDIF function uses two mutually
independent characteristics to which magnitudes of the three fundamental
frequency RMS differential currents are compared at each execution of the
differential protection function. These two characteristics divide, each of them
independently, the operate current – restrain current plane into two regions: the
operate (trip) region and the restrain (block) region, as shown in figure 51. Two
kinds of protection are obtained:
• the non-stabilized (instantaneous unrestrained) differential protection
• the stabilized differential protection
The non-stabilized (instantaneous) differential protection is used for very high
differential currents, where it must be beyond any doubt, that the fault is internal.
This limit, (defined by the setting UnrestrainedLimit), is a constant, not
proportional the bias (restrain) current. No harmonic or any other restrain is applied
to this limit, which is, therefore, called the unrestrained limit. The reset ratio of the
unrestrained characteristic is equal to 0.95.
The stabilized differential protection applies a differential (operate) current, and the
common bias (restrain) current, on the operate-restrain characteristic, as shown in
figure 51. Here, the actual limit, where the protection can operate, is dependent on
the bias (restrain) current. The operate value, is stabilized by the bias current. This
operate – restrain characteristic is represented by a double-slope, double-breakpoint
characteristic. The restrained characteristic is determined by the following 5 settings:
• IdMin (Sensitivity in section 1, set as multiple of generator rated current)
• EndSection1 (End of section 1, set as multiple of generator rated current)
• EndSection2 (End of section 2, set as multiple of generator rated current)
• SlopeSection2 (Slope in section 2 of the characteristic, set in percent)
• SlopeSection3 (Slope in section 3 of the characteristic, set in percent)
IL1t
Idiff = 0
en07000019-2.vsd
Section 5
Differential protection
IL1n
105