ABB 650 series, Technical Manual

Page: 145 / 778

current – restrain current plane into two regions: the operate (trip) region and the restrain
(block) region, as shown in figure
54 . 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 dependent on 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
54 . 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)
100%
Ioperate
slope
Irestrain
D
= D ×
EQUATION1246 V1 EN
(Equation 32)
Note that both slopes are calculated from the characteristics break points.
The operate-restrain characteristic is tailor-made, in other words, it can be constructed by
the user. A default operate-restrain characteristic is suggested which gives acceptably
good results in a majority of applications. The operate-restrain characteristic has in
principle three sections with a section-wise proportional dependence of the operate value
to the common restrain (bias) current. The reset ratio is in all parts of the characteristic
equal to 0.95.
Section 1 is the most sensitive part on the characteristic. In section 1, normal currents flow
through the protected circuit and its current transformers, and risk for higher false
differential currents is low. With generators, the only cause of small false differential
1MRK 502 048-UUS A
Section 6
Differential protection
139
Technical manual