ABB RELION REX640 Technical Manual page 1029

1MRS759142 C
REX640
Technical Manual
I I UNB I A
_ _
∠ = ∠ − ∠
unb
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In a three-phase star-connected capacitor bank circuit, there may be some amount of
natural unbalance current flowing through the neutral, which is primarily due to
capacitor manufacturing tolerances. The natural unbalance current must be
compensated for before using the measured unbalance current for the function
operation. The natural unbalance current needs to be recorded when there is no fault
in the capacitor banks, and it is initiated through the command Record unbalance,
available under menu path Control/CUBPTOC. By selecting Record unbalance with
value "Record", the measured unbalance current
unbalance current
I
NatUnb
recorded natural unbalance current I_AMPL_NAT and I_ANGL_NAT are available
in the monitored data view.
Once the natural unbalance current is recorded during further executions of the
function, the natural unbalance current is subtracted from the measured unbalance
current to obtain the compensated unbalance current
I
Unb
554.
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Figure 554: Natural unbalance compensation. (a) Healthy condition when the
natural unbalance is recorded (b) Unbalance compensation during
faulty conditions
The natural unbalance current compensation is enabled using the setting Natural
Comp Enable. If Natural Comp Enable is set to "FALSE", the unbalance current is not
compensated. If Natural Comp Enable is set to "TRUE", the compensated unbalance
current is calculated based on the equation.
I I I
CompUnb = Unb − NatUnb
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and is stored as a reference. The amplitude and angle of the
Section 4
Protection functions
(Equation 240)
I is considered as the natural
Unb
I
as shown in
CompUnb
(Equation 241)
Figure
1023