Operation principle and hardware description 41
Fast power off (FPO) circuit
The fast power off (FPO) circuit is powered from the 24 V DC circuit which is connected
directly to the FPO switch (-0S20). The FPO switch is a locking type (twist release) push
button that has one normally open (N.O.) contact and one normally closed (N.C.) contact.
The N.C. contact connects to the main contactor enable relays (-xK11) through the
contactor control duplication relay output of the main control unit. All of the contactor
control signals from the internal PLC connect to the contactor coils through the enable
relay (-xK11) contacts. Each power section has an enable relay, which all are in parallel
downstream of the common FPO switch.
The N.O. contact of the FPO switch connects to the shunt trip coil of the auxiliary voltage
supply transformer protection switch (-xQ11).
Optionally, the inverter can have AC breakers (-xQ4) with an undervoltage release. The
undervoltage release opens the breaker or prevents the breaker from closing, if the supply
voltage from the undervoltage release coil disconnects, that is, if the FPO switch is
pushed.
In addition to the interlockings on the hardware side, the software prevents inverter
operation when the FPO switch is pushed.
Startup after FPO activation
After an FPO event, all of the power sources (main AC, main DC and internal auxiliary
supply) are disconnected by the contactors and/or by the circuit breakers. Before startup,
the FPO switch must be released. Otherwise the auxiliary voltage supply trips again when
the 24 V DC power supply starts to supply power.
The start-up procedure:
1. Set the Run/Stop switch to STOP and release the FPO switch.
2. Set the auxiliary supply transformer protection switches (-xQ11) in all AC sections to
ON. This allows the power supplies and the control system to wake up and start the
communication, measurements and other inverter functions.
3. Close all of the panels, set the Run/Stop switch to RUN and reset any faults if
necessary.
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