in the "direct" position. Reversed reactance
compensation is obtained by placing it in the
"reverse" position, which reverses the po
larity of the reactance element.
"Reverse reactance" compensation is a
method used to reduce the circulating cur
rent that might flow when two or more trans
formers are paralleled. It is a requirement
of ASA Standards C57.12-37.236.1. Instead
of running toward opposite extreme posi
tions, tap changers having "reverse re
actance" compensation tend to move toward
whatever positions cause the least amount of
circulating current to flow. This is accom
plished at some sacrifice of normal line
drop compensation, but is generally satis
factory when units paralleled are not located
in close proximity to each other, or where
the supply is from different sources.
4. Line
The
final settings on the line-drop compensator
are usually made by field adjustments, but
if the data on the particular line is known,
the curves in Figs. 12 and 13 may be used,
and initial values calculated.
The initial line-drop compensator set
tings can be derived by the use of the follow
ing expressions:
Dial Setting for Resistance Compensation =
Nc T
5
X
•
•
X
RL
X
d
X
n.
NPOT
Dial Setting for Reactance Compensation =
N
5 x
C.T. x XL x d x n.
NPOT
Where
Nc. T. = main current transformer ratio
primary current
NPOT =
=
potential transformer ratio
primary voltage
Page 15
RL
resistance per conductor from
unit to load center, in ohms per
mile.
=
inductive reactance per conduc
tor from unit to load center, in
ohms per mile.
d
miles from unit to load center.
n
120/balance voltage setting.
A typical three-phase example is as fol
lows:
a
b
c
0
.,...__
4'
-
0
-
4'-0
I
8,
I
500,000 CM copper conductor, with flat
spacing above.
Line Voltage = 12000 volts
Main Current Transformer Ratio= 600/5
Potential Transformer Ratio = 6928/120
Distance from unit to load center 3 .5 miles.
Balance voltage setting = 117 volts.
A unit energizes a typical distribution
circuit whose characteristics are given
above. Determining the constants for the
circuit on a per phase basis,
From Figure 12:
R = 0.12 ohms per mile.
From Figure 13:
D = 1.26x4 = 5.04 feet.
XL = 0.64 ohms per mile.
The line drop compensator resistance set
ting is:
5
X
120
_
6928/120
X
0.12
X
3.5
X
117 - 4.47
The line drop compensator reactance set
ting is:
5
X
600/5
120
6928/120
X
0.64
X
3.5
X
=
23.9
These settings may be adjusted as found
necessary as shown by load center voltage
measurements.
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