ABB RED650 Applications Manual page 95

1MRK 505 393-UEN B
Line differential protection RED650 2.2 IEC
Application manual
The positive sequence line charging current is calculated according to equation 12.
U
=
=
I
Ch arg e
3
×
XC 1
EQUATION1417 V2 EN-US
where:
U is system voltage
X is capacitive positive sequence reactance of the line
C1
f is system frequency
C is positive sequence line capacitance
1
If the charging current compensation is enabled, the setting of IdMin must be:
IdMin ≥ 1.2 × ICharge, concidering some margin in the setting. If the charging
current compensation is disabled, the setting of IdMin must be IdMin ≥ 2.5 ×
ICharge. In many cases, the charging current is quite small, which makes the lower
limit of the setting range, that is 20% of IBase the practical limit of sensitivity.
When a power transformer is included in the protected zone, the setting of IdMin
shall be the highest of recommendations considering charging current as described
above and 0.3 × IBase.
IdMinHigh
The IdMinHigh setting is a multiple of IBase and used to temporarily decrease
sensitivity in situations when the line is energized.
Energizing a line can cause transient charging currents to appear. These currents
are pure differential currents, but as they are rich in harmonics, they can partly be
measured by the differential protection, which in this case measures the Fourier-
filtered differential current. Desensitizing the differential protection in this situation
by using IdMinHigh instead of IdMin is a safety precaution, and a setting of 1.00 ×
IBase should be suitable in most cases.
If there is a power transformer included in the protected zone, energizing the line
means that the transformer is energized at the same time. If the transformer
nominal current is more than 50% of IBase, IdMinHigh is recommended to be set
at 2.00 × IBase, otherwise it can be kept at 1.00 × IBase.
Switching of a transformer inside the protected zone does not normally occur. If the
transformer is equipped with a breaker on the HV side, it would most probably not
be included in the protected zone. However, tap transformers are sometimes
connected with a disconnector on the HV side, and the normal procedure is then to
energize the transformer with the disconnector. In such cases, where the tap (shunt)
power transformer's power rating is relatively small in comparison to the normal
load of the circuit, connecting the tap power transformer to the voltage source, that
U
1
×
3
p
×
2 f C
1
Section 6
Differential protection
(Equation 12)
M12541-165 v5
89