Voltage Sensing And Correction; Voltage Limit Control; Reverse Power Flow - Siemens MJ-5 Installation And Operation Manual

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6.3 Voltage Sensing and Correction

Voltage Sensing
The control program monitors the regulator output voltage
signal from one of the following sources:
• a voltage transformer, "VT" [also known as a potential
transformer or "PT"], integral to the regulator, which is
turns-ratio corrected to deliver 120 VAC at the nominal
system voltage.
•
the utility winding of the regulator, which generally
requires correction to deliver 120 VAC at the nominal
system voltage.
The control program senses regulator output (i.e., "load")
voltage depending on power flow operating mode (see
Table 6.2):
Table 6.2 Regulator Output Voltage Sensing
Since the control program routinely senses both the voltage
transformer and the utility winding, it is a straightforward
process for the control program to select which source is
"active" (depending on power flow mode and regulator
design).
If a voltage transformer is not installed on the regulator,
then the control will calculate the voltage on that side of the
regulator based on the utility winding voltage and tap
position.
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.

6.4 Voltage Limit Control

The voltage limit control (VLC) function monitors
regulator output voltage and compares it to maximum and
minimum 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
outside of the VLC limit range, the tap position is
incremented/ decremented until the voltage is within the
defined VLC range.
Siemens Industry, Inc.
MJ-5 Control Panel Automatic Mode
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
immediately, (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
algorithm 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 outside
prescribed limits.

6.5 Reverse Power Flow

Systems where power flow reversals occur have unique
regulation control requirements. For systems of this type,
the MJ-5 senses the reversal and adjusts its operation
accordingly. When power flow direction changes, the
MJ-5 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:
6.5.1 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 current
represents the component of the current which is in phase
with the load voltage signal.
The MJ-5 Control Panel indicates forward power flow
when the real power (kW) is positive and the real
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