6
MJ-XL Control Panel Automatic Mode
42
Siemens Power Transmission & Distribution, Inc.
Software Voltage Measurement Correction
The monitored output voltage is scaled appropriately to
the nominal basis voltage in two steps. The first voltage
scaling is performed through a step-down transformer.
The second scaling takes place in the software algorithm
after the voltage is converted to a numeric representation.
Software scaling corrects the nominal input voltage level
to the numeric representation of the basis voltage. The
basis voltage value is user configurable. The choices are
115, 120 and 125 V. The default basis voltage is 120 V.
6.5
Voltage Limit Control
The voltage limit control (VLC) function monitors regulator
output voltage and compares it to maximum and mini-
mum voltage limit set points. If the VLC function is active,
the control program inhibits tap changes that would cause
the voltage to go above the VLC upper or below the VLC
lower voltage. If VLC is activated while the voltage is out-
side of the VLC limit range, the tap position is incre-
mented/decremented until the voltage is within the
defined VLC range.
Limiting and/or runback may occur if one of the following
happens:
1.
The source voltage changes dramatically,
2.
The load on the regulator changes significantly, or
3.
The VLC function is enabled when the limit conditions
are exceeded.
Note:
The VLC function uses the Vld (i.e., the load
voltage at the regulator) to determine whether
or not to limit or runback. The VLC algorithm
senses one regulator bushing for forward
power flow, and the other regulator bushing for
reverse power flow. (See Forward/Reverse
Operation modes, below).
The maximum voltage limit set point is adjustable from
106.0 volts to 134.0 volts in 0.1 volt steps. The minimum
voltage limit set point is adjustable from 106.0 volts to
134.0 volts in 0.1 volt steps. The control program will not
allow these limits to be set so that they overlap one
another.
If regulator output voltage falls outside limits, VLC acts to
return regulator voltage within limits. Return begins imme-
diately, (i.e., without regard to the time delay setting),
while avoiding a hunting condition, (i.e., VLC will not force
a tap change which would activate the basic control algo-
rithm and force the regulator outside limits once again).
VLC activates when load voltage is within one volt of the
upper/lower limit to prevent the voltage from going out-
side prescribed limits.
6.6
Reverse Power Flow
Systems where power flow reversals occur have unique
regulation control requirements. For systems of this type,
the MJ-X
L
senses the reversal and adjusts its operation
accordingly. When power flow direction changes, the
MJ-X
L
control algorithm takes the following factors into
account:
1.
Power Flow direction,
2.
Forward or Reverse operating mode, and
3.
Power Flow Mode as selected from the <
CONFIG-
URE
> Menu. These factors are discussed in greater
detail below:
6.6.1
Power Flow Direction
The microprocessor determines power flow direction by
continually monitoring the real power (kW) reading and
the real component of current. The real component of cur-
rent represents the component of the current which is in
phase with the load voltage signal.
The MJ-X
L
Control Panel indicates forward power flow
when the real power (kW) is positive and the real compo-
nent of current is greater than 1% of the full scale current.
Reverse power flow is indicated when the real power is
negative and the real component of current is greater than
1% of the full scale current. To avoid excessive switching
between Forward and Reverse Operation (defined
below), the microprocessor delays five seconds after the
occurrence of a current flow reversal before switching
from forward to reverse, and vice-versa.
As an example, assume current is flowing in the forward
direction. When load current magnitude drops through
zero and increases to a magnitude that exceeds 1% rated
current in the reverse direction, the microprocessor
delays five seconds and then considers reverse power
flow to be in effect. Reverse power flow remains in effect
until current direction becomes forward once again and its
magnitude exceeds 1% of rated current in the positive
direction for five seconds. At that time, the microproces-
sor considers forward power flow conditions to have
resumed.
Power flow direction is indicated on many of the metering
screens. “F” indicates forward power flow, “R” indicates
reverse power flow. “F” is shown when: kW > 0 and
|
I
real| > 1% (of full scale) for 5 seconds minimum. “R” is
shown when: kW < 0 and |
I
real | > 1% (of full scale) for 5
seconds minimum. If the real current magnitude is below
the 1% threshold, the last known power flow direction
remains in effect. If the real current magnitude is below
the 1% threshold after power up or unit reset, the MJ-X
L
defaults to forward power flow operation.
It is possible for Reverse Power Flow conditions to occur
in regulators with no source-side voltage input (e.g., no
“Source-Side PT” for Siemens Inverted Regulators.), In
this case, the MJ-X
L
uses the calculated source-side volt-
age for performing reverse power flow regulation. See
Summary of Contents for MJ-XL
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