Individual capacitor step circuit breakers provide shunt capacitor bank switching. There are two modes of operation: manual and automatic.
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.
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.