ABB RELION REX640 Technical Manual page 1555

1MRS759142 C
REX640
Technical Manual
Line drop V Ris U %
[ ]
=
r
Line dro p p V React U
[ ]
=
x
GUID-B57BF65C-BC25-4033-87B0-CC54825DAD46 V1 EN
I Nominal primary current of the CT
CT_n1
U Nominal primary voltage of the VT (phase-to-phase voltage)
VT_n1
R Resistance of the line, Ω/phase
X Reactance of the line, Ω/phase
The general LDC equation can be calculated.
U % cos
I ϕ
(
[ ]
r
L
U
= ⋅
z
I
CT _ n 1
GUID-8EA49BED-6AE9-4DE1-99B2-25F807CEF682 V2 EN
I Average of the currents I_A, I_B and I_C
L
I Nominal primary current of the CT
CT_n1
Line drop V Ris
U Setting
r
U Setting Line drop V React
x
φ Phase angle between U_A and I_A (ANGL_UA_IA)
By default, LDC is not active, but it can be activated by setting LDC enable to "True".
To keep the LDC term within acceptable limits in all situations, OL5ATCC has a
setting parameter LDC limit, which has a default value of 0.10 xU
maximum value for U
z
If more than one line is connected to the LV busbar, the equivalent impedance is
calculated and given as a parameter setting as shown in
electrical circuit for calculating LDC. For example, if there are N number of identical
lines with identical loads in the substation, the R and X values needed for the settings
Line drop V React and Line drop V Ris are obtained by dividing the resistance and the
reactance of one line by N. Because the voltage drop is different in lines with different
impedances and load currents, it is necessary to make a compromise when setting the
Line drop V React and Line drop V Ris settings. Raising the voltage in the point of the
lowest voltage must not lead to overvoltage elsewhere.
By default, LDC is effective only on the normal active power flow direction. If the
active power flow in the transformer turns opposite, that is, from the regulated side
towards the system in the upper level, the LDC term is ignored, that is, it is set to zero.
In such a case, it is assumed that the feeding units at the regulated side of the
transformers maintain proper voltage levels. This can cause a conflict if the
transformer tries to reduce the voltage at the substation. Additionally, it is difficult to
I R
3
⋅ ⋅
CT n
_ 1
100 %U
[
= ⋅
U
VT _ n 1
I X
3
⋅ ⋅
CT _ n 1
% 100
= ⋅
U
VT _ n 1
U % sin
ϕ
)
[ ]
+
x
xU
[ ]
n
100
in Equation 338.
Section 9
Control functions
]
n
U
%
[ ]
n
. This gives the
n
Figure 813 for the equivalent
(Equation 339)
(Equation 340)
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