ABB Relion 670 Series Technical Reference Manual page 139

1MRK505222-UUS C
Technical reference manual
•
A pickup in one phase, gives a trip under the condition that the content of 2
th
5 harmonic is below the set level for these harmonics. Otherwise it is blocked as
long as the harmonic is above the set level. However, when a line is energized the
current setting value IdMinHigh is used. Effectively this means that the line A-B-C
in figure 52 forms the characteristic.
• Current values above the unrestrained limit gives a trip irrespective of any
presence of harmonics.
• Classification of a fault as internal by the negative sequence fault discriminator,
will give a trip under the condition that a pickup has occurred in that phase. This
means that any harmonic blocking is then overridden. However, occurrence of
harmonics at the same time as the differential current is below the level
IdMinHigh, will block a trip even though the fault is classified as internal. This
latter condition is to prevent unwanted trips when energizing a line tap transformer.
• Classification of a fault as external by the negative sequence fault discriminator
will cause IdMinHigh to be used as the lower limit for the restrained characteristic
according to figure 52. Cross blocking will also be activated in this situation.
Compensation for charging currents can be selected active or not by setting
ChargCurEnableYes or No. The compensation works such that the fundamental
frequency differential current that is measured under steady state undisturbed
conditions, is identified and then subtracted making the resulting differential current
zero (or close to zero). This action is made separately for each phase. The magnitude of
the subtracted pre-fault currents in Amperes can be read at any time as the service
value ICHARGE.
Values of the pre-fault differential currents are not updated under disturbance
conditions. The updating process is resumed 50 ms after normal conditions have been
restored. Normal conditions are only considered if there are no pickup signals, neither
internal nor external fault is declared, the power system is symmetrical and so on.
It is thus obvious that the change in charging current that the fault causes by decreasing
the system voltage is not considered in the algorithm, a matter that is further discussed
in the application manual.
Note that all small pre-fault differential currents are subtracted, no matter what their
origin. Besides the true charging currents, the following currents are eliminated:
• Small differential currents due to small errors (inequalities) of current transformers.
• Small differential currents because of off-nominal load tap changer positions when
a power transformer is included in the protected zone.
• Load currents of tap loads included in the protected zone.
Section 5
Differential protection
nd
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