ABB RELION 650 Series Applications Manual page 90

Section 6
Differential protection
=
I
Ch arg e
EQUATION1417 V2 EN-US
where:
U
X
C1
f
C
1
If the charging current compensation is enabled, the setting of
ICharge, concidering some margin in the setting. If the charging current compensation is
disabled, the setting of
current is quite small, which makes the lower limit of the setting range, that is 20% of
the practical limit of sensitivity.
When a power transformer is included in the protected zone, the setting of
highest of recommendations considering charging current as described above and 0.3 ×
IdMinHigh
IdMinHigh setting is a multiple of IBase and used to temporarily decrease sensitivity in
The
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
is a safety precaution, and a setting of 1.00 ×
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 .
1.00 ×
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 is, to the protected line circuit, does usually not result
in inrush currents high enough to be detected by the differential protection. This is especially
the case if the tap power transformer is connected at a junction somewhere towards the
middle of the protected line circuit, because the inrush phenomenon is effectively prevented in
such cases. If a start signal is nevertheless issued by the differential protection, then the 2
harmonic will prevent an unwanted trip. Observe that the harmonic inhibit algorithm is active
as long as the bias current is below 125% of the base current
transformer included in the protected line circuit, then the harmonic inhibit algorithm is
permanently activated, and can only be overridden if an internal fault has been detected.)
tIdMinHigh
This setting defines the time that
circuit has been connected to the power source. If a power transformer is included in the
84
U U
=
3
×
XC 1
×
3
p
2
is system voltage
is capacitive positive sequence reactance of the line
is system frequency
is positive sequence line capacitance
IdMin must be IdMin ≥ 2.5 × ICharge. In many cases, the charging
IBase , IdMinHigh is recommended to be set at 2.00 × IBase , otherwise it can be kept at
1
×
f C
1
IBase should be suitable in most cases.
IdMinHigh will be active after the previously dead protected
1MRK 505 363-UEN A
(Equation 15)
IdMin must be: IdMin ≥ 1.2 ×
IdMin shall be the
IdMinHigh instead of IdMin
IBase . (If there is an in-line power
Application manual
IBase
IBase .
M12541-165 v5
nd
M12541-177 v4