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
55. 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-US (Equation 27)
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 currents in this section
can be tolerances of the current transformers used on both sides of the protected generator.
Slope in section 1 is always zero percent. Normally, with the protected machine at rated load, the
restrain, bias current will be around 1 p.u., that is, equal to the machine rated current.
In section 2, a certain minor slope is introduced which is supposed to cope with false differential
currents proportional to higher than normal currents through the current transformers.
The more pronounced slope in section 3 is designed to result in a higher tolerance to substantial
current transformer saturation at high through-fault currents, which can be expected in this
section.
Temporarily decreased sensitivity of differential protection is activated if the binary input
DESENSIT is (temporarily) set to 1 (TRUE). In this case, a new, separate limit is superposed to the
otherwise unchanged operate-bias characteristic. This limit is called
TempIdMin
and it is available
as a setting. The value of the setting
TempIdMin
must be given as a multiple of the setting
IdMin
.
In this case no trip command can be issued if all fundamental frequency differential currents are
below the value of the setting
TempIdMin
.
AddTripDelay
: If the input DESENSIT is activated also the operation time of the protection
function can be increased by using the setting
AddTripDelay
.
Section 6 1MRK 502 066-UUS B
Differential protection
156
Technical manual
To Next Page
Loading...
