Theoryof Operation 19
2.2.5 Static Bypass
Astatic bypass is an integral part of the UPS System Control Cabinet(SCC). Refer to Figure 7. The static
bypassconsists of solid-state switching devices working in conjunctionwith the motor-operated System Bypass
Breaker(SBB). An automatic transfer controlcircuit senses the status of the operator controls, UPS logicsignals
andalarm messages, and critical bus operating conditions. If the UPS modulescan nolonger supply the critical
load,the static bypass switch (in conjunction with the SBB and UPS Output Breaker) automatically transfersthe
criticalload to the bypass source without interruption.
Figure 7 SystemC ontrol Cabinet (SCC) Block Diagram
Fuse Protection
Thestatic bypass switch path uses two back-to-back SCRs per phase. Eachphase is individually protected by a
fusesized to clear only in the event of a catastrophic fault.This is a more reliable method than depending on
externalprotection devices. The fuses are in the circuit to protectthe critical bus distribution equipment against
catastrophicfaults. The static switch SCRs are oversizedto easily handle any current surges that may blow the
fuses.
Shorted SCR Monitoring
Thestatic bypass system has redundant shorted SCR sensingcircuits and disconnects. This will prevent UPS out-
putpower backfeed to the distribution system even if two component failures existsimultaneously. If a shorted
SCRis detected, the static bypass switch is isolated and annunciatedat the SCC control panel, and the critical
loadremains on UPS output power.
StaticSwitch Isolation
Themotor-operated system bypass circuit breaker (SBB),wired in parallel with the static switch, automatically
closesin approximately 200 milliseconds aftert heload is transferred to the bypass power source, removingthe
staticswitch from the power flow.
UPSOutput
Gate
Driver
Sync &
Transfer
Control
Power
Supply
Voltage
Adjust
Alarms
Static
Bypass
Switch
StaticBypassBreaker
Transfer
Control
StaticSwitch
Disconnects
BypassLine
UPS
System
Modules
ToCriti cal
Load
System
Monitor
DISCONTINUED
PROD U C T