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1MRK 505 370-UEN D
These three quantities can be easily calculated numerically from the raw sample values (that is,
twenty times within each power system cycle in the IED) from all analog CT inputs connected
to the differential zone. At the same time, they have extremely valuable physical meaning,
which clearly describes the condition of the protected zone during all operating conditions.
By using the properties of only these three quantities, a new patented differential algorithm
has been formed in the IED. This differential algorithm is completely stable for all external
faults. All problems caused by the non-linearity of the CTs are solved in an innovative numerical
way. Meanwhile, very fast tripping time, typically 11 ms, can be commonly obtained for heavy
internal faults.
Discriminating zones in the IED includes a sensitive operational level. This sensitive
operational level is designed to be able to detect busbar earth faults in low impedance earth
power systems (that is, power systems where the earth-fault current is limited to a certain
level, typically between 300 A and 2000 A by neutral point reactor or resistor) or for some
other special applications where increased sensitivity is required. Operation and operating
characteristic of the sensitive differential protection can be set independently from the
operating characteristic of the main differential protection. The sensitive differential level is
blocked as soon as the total incoming current exceeds the pre-set level. By appropriate setting
then it can be insured that this sensitive level is blocked for external phase-to-phase or three-
phase faults, which can cause CT saturation. Comparison between these two characteristics is
shown in Figure 40.
DiffOperLevel
SensOperLevel
IEC06000142 V2 EN-US
Figure 40: Differential protection operation characteristic
where
I
in
I
d
s = 0.53
Additionally the sensitive differential protection can be time delayed and it must be externally
enabled by a binary signal (that is, from external open delta VT overvoltage relay or power
transformer neutral point overcurrent relay).
Busbar protection REB670
Application manual
s=0.53
RMS value of the incoming current into the differential protection zone
RMS value of the differential current from the differential protection zone
operating slope for the differential function (fixed in the algorithm).
Sensitive
differential
protection
Operate
region
Differential protection
operation characteristic
SensIinBlock
I
in
Differential protection
SEMOD127483-4 v4
[Primary Amps]
IEC06000142-2-en.vsdx
Section 6
95
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