Overview Of Automatic Control Algorithm; Remote (Communications Module) Override Of Automatic Control; Voltage Sensing And Correction - Siemens MJ-XL Installation And Operation Manual

6.2 Overview of Automatic Control
Algorithm
The automatic control algorithm maintains the output volt-
age within its prescribed limits while following a control
hierarchy.
Normal algorithm operation is as follows. When voltage
falls outside the allowed range (defined by voltage level
set point ± ½ bandwidth), the delay timer is started. If volt-
age is out of range at the end of the delay period, the tap
change motor is energized. When the voltage is back
within range, the tap change motor is deactivated. (Hys-
teresis is included to prevent oscillations.)
In automatic mode, the control program processor con-
trols the tap changer motor. The control algorithm takes
the following into account:
1. Calculated or monitored regulator output voltage
2. Voltage-level set point
3. Bandwidth set point
4. Resistance line drop compensation set point
5. Reactance line drop compensation set point
6. Voltage limit control set points
7. Voltage reduction control setpoints
8. Monitored load current
9. Load power factor
10. Power flow operating mode
11. Power flow direction.
Because coinciding events might introduce conflicts, an
operational hierarchy must be established. This hierarchy
is maintained when the unit is under automatic control.
Manual mode always takes precedence over automatic
control.
Conflicts are resolved according to the following priority
scheme (from highest to lowest):
1. Automatic tap change inhibit (implemented with ter-
minal strip jumpers or Comm Link command).
2. Voltage Limit Control (defined in regulator settings).
3. Current inhibit (determined by %I Threshold of con-
figuration setting; as % of full scale CT rating).
4. Voltage Reduction Control.
5. "Normal" regulator control operation.
Siemens Power Transmission & Distribution, Inc.
6
MJ-XL Control Panel Automatic Mode
6.3 Remote (Communications
Module) Override of Automatic
Control
Place the Remote/Auto/Manual switch in the Remote
position to enable remote override of local automatic oper-
ation. A remote operator can request a tap raise or lower
via the communications link. When the remote operator
initiates the tap raise/lower, the MJ-X
"Pseudo-Manual" operating mode. The Pseudo-Manual
alert condition is activated, and local automatic operations
are disabled until the remote-commanded tap raise/lower
is completed.
L
The MJ-X Control Panel remains in the Pseudo-Manual
mode for a predefined time period after receipt of the last
remote tap raise/lower command. When the time period
has elapsed, the MJ-X
automatic control operations.
[To block Local Automatic control, assert Automatic Inhibit
(via Terminal Strip or Comm Link).]
6.4

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 poten-
tial 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):
Regulator Output Voltage Sensing
Table 6.2
Regulator Design
ANSI Type
A (Straight)
B (Inverted)
Since the control program routinely senses both the volt-
age transformer and the utility winding, it is a straightfor-
ward process for the control program to select which
source is "active" (depending on power flow mode and
regulator design).
L
enables its
L
Control Panel resumes its normal
Winding Used for Voltage Sensing
Forward Power Reverse Power
Flow Operation Flow Operation
Voltage Utility winding
transformer
Utility winding Voltage
transformer
41