Individual capacitor step circuit breakers provide shunt
capacitor bank switching. There are two modes of operation: manual and
automatic.
Manual Mode
In the manual mode, operators switch capacitor banks from
the control room through step control switches installed on the switchboards.
The following permissive conditions must be met before a capacitor step can be
switched on or off by the manual control switches.
· The capacitor step
must be in manual mode.
· The circuit breaker
must be open for at least five minutes before it can be reclosed.
This allows the capacitors to be discharged to a safe
voltage level through their internal discharge device. If the breaker is not
open for five minutes, the programmable logic controller blocks the
close circuit and prevents the operator from closing the
breaker.
Automatic Mode
In the automatic mode, the PLC controls the switching of the
shunt capacitor steps. The PLC is programmed to perform voltage control and
event recording, record step operations, balance switching operations, and
communicate with a programmable graphics interface (PGI) located in the control
room.
The PGI is used to adjust voltage set points, and to view
the status of capacitor step devices and bank alarms.
To perform automatic step insertion, the PLC compares the
control voltage set-point with the measured system bus voltage. System voltage
measurement is performed using two transducers.
Their difference output of these transducers is compared
against set thresholds before the system voltage measurement is accepted as
valid. A voltage dead-band is also part of the step switching settings. The
dead-band setting is greater than the voltage change caused by the addition or
removal of one capacitor step, to avoid hunting by the system. The following
must be satisfied in order to insert a capacitor step:
· The capacitor step
must be in automatic mode.
· No capacitor step
switching has been performed for a two-minute time period.
· Before a step is
closed, it must be de-energized for five minutes.
· The main circuit
breaker is closed and there is voltage on the bus.
The order of capacitor step insertion is sequenced to
balance the number of operations for each step. If all steps have the same
number of operations, the insertion is performed in a numerical ascending
order. A "Step x Fail to Close" alarm, and a software latch is set if
a capacitor step fails to close in five seconds.
The software latch prevents further operations, until the
problem has been resolved and the operator resets the capacitor step switching
via the PGI. Automatic step removal is performed in a similar fashion as step
insertion. The PLC compares the system voltage with the control voltage
set-point.
If the system voltage is higher than the set-point voltage
plus the dead-band setting, the PLC opens the capacitor step in a " First
in – First out" order provided no switching action took place in the last
two minutes. A "Step x Fail to
Open" alarm, and a software latch is set if a capacitor step fails to open
in five seconds. The software latch prevents further operations, until the
problem has been resolved and the operator resets the capacitor step switching
via the PGI.
Rapid insertion of capacitor steps can help system
performance in the event of a major disturbance. The PLC monitors the
rate-of-change of voltage to perform a rapid insertion of a capacitor step. If
the rate-of-change of voltage exceeds four kV per second, and the system
voltage falls ten percent below nominal, the PLC issues the commands to insert
capacitor steps in two-second intervals. Rapid removal is also performed in the
event the system voltage exceeds normal voltage by five percent. Capacitor
steps are removed every two-seconds until the system voltage is less than five
percent above normal voltage.
A number of different pages on the PGI monitor the operation
of the capacitor bank installation. The PGI provides the following screens:
· The System Status
screen displays the operating status of the 230 kV capacitor steps and
the analog meter values for each step. The Single Line
Diagram screen displays the station single line diagram of the substation, the
open/close counters, and "Out-of-Service" capacitor steps.
· The Voltage
Set-point screen displays the current voltage set-points, and dead-bands. In
addition, the operator can make changes of the voltage set-points from this
screen.
· The Alarm screen
displays up to 28 alarm messages from the 230 kV capacitor banks, and relays.
· The Sequence of
Events screen provides access to sequence of events recording for 48 points for
the last 50 events.
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