ABB Relion 670 series, Руководство по применению

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arg
1
1
1
2
3
3
Ch e
U U
I
XC
f C
p
=
×
×
=
×
EQUATION1417 V2 EN-US
(Equation 26)
where:
U is system voltage
X
C1
is capacitive positive sequence reactance of the line
f is system frequency
C
1
is positive sequence line capacitance
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
M12541-165 v5
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 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
nd
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
IBase. (If there is an in-line power
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
M12541-177 v4
This setting defines the time that
IdMinHigh will be active after the previously dead protected
circuit has been connected to the power source. If a power transformer is included in the
Section 6 1MRK 505 343-UEN B
Differential protection
126
Application manual