Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
124
The harmonic restraint operates individually per phase. However, it is also possible – as it is for the inrush re-
straint – to set the protection such that not only the phase with harmonics content in excess of the permissible
value is stabilized but also the other phases of the differential stage IDIFF> are blocked ("cross-block" function).
The duration for which the cross-block function is to remain operative after differential current overshoot, is set
at address 2077 CROSSB. n.HARM. The setting is in multiples of an cycle. When set to 0 Cycle (default set-
ting), the protection can initiate a trip when the transformer is switched onto a single-phase fault, even if a high
harmonic content is present in another phase. When set to ∞, the cross-block function is always effective.
If the differential current exceeds a multiple of the rated transformer current specified at address 2078
IDIFFmax n.HM, no nth-harmonic restraint takes place.
Tripping Characteristic
The parameters of the tripping characteristic are set in addresses 2021 to 2044. The meaning of the parame-
ters can be seen in the following figure. The numerical values at the characteristic branches are the parameter
addresses.
Address 2021 I-DIFF> is the pickup value for the differential current. This is the total fault current flowing in
the protection area, regardless of the way it is distributed between the windings of the protected transformer.
The pickup value is referred to the rated current corresponding to the rated apparent power of the transformer.
For transformers, the setting should be between 0.2 and 0.4. It should be checked during commissioning that
the selected pickup value is at least twice the maximum differential current present in steady-state operation.
In addition to the pickup threshold I-DIFF>, a second pickup threshold is introduced. If this threshold (2031
I-DIFF>>) is exceeded, tripping is initiated regardless of the magnitude of the restraint current (unstabilized
high-speed trip stage). This stage must be set higher than the I-DIFF> stage. As a guide: greater than the
reciprocal value of the transformer´s relative short-circuit value 1/u
k
times the rated transformer current.
The tripping characteristic forms two more branches (see the following figure). Address 2041 SLOPE 1 deter-
mines the slope of the first branch, whose starting point is specified in the parameter 2042 BASE POINT 1.
This branch covers current-proportional error currents. These are mainly transformation errors of the main CTs
and, especially, the differential currents which may occur in the final tap changer positions due to a possible
transformer regulation range. This branch of the characteristic limits the stabilization area. The preset slope of
0.25 should be sufficient for regulating ranges up to 20 %. If the transformer has a larger regulated range, the
slope must be increased accordingly.
Figure 2-46 Parameters Determining the Shape of the Tripping Characteristic
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