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Theory of Operation 19
2.2.5 Static Bypass
A static bypass is an integral part of the UPS System Control Cabinet (SCC). Refer to Figure 7. The s tatic
bypass consists of solid-state switching devices working in conjunction with the motor-operated System Bypass
Breaker (SBB). An automatic transfer control circuit senses the status of the operator controls, UPS logic signals
and alarm messages, and critical bus operating conditions. If the UPS modules can no longer supply the critical
load, the static bypass switch (in conjunction with the SBB and UPS Output Breaker) automatically transfers the
critical load to the bypass source without interruption.
Figure 7 System Control Cabinet (SCC) Block Diagram
Fuse Protection
The static bypass switch path uses two back-to-back SCRs per phase. Each phase is individually protected by a
fuse sized to clear only in the event of a catastrophic fault. This is a more reliable method than depending on
external protection devices. The fuses are in the circuit to protect the critical bus distribution equipment against
catastrophic faults. The static switch SCRs are oversized to easily ha ndle any current surges that may blow the
fuses.
Shorted SCR Monitoring
The static bypass system has redundant shorted SCR sensing circuits and disconnects. This will prevent UPS out-
put power backfeed to the distribution system even if two component failures exist simultaneously. If a shorted
SCR is detected, the static bypass switch is isolated and annunciated at the SCC control panel, and the critical
load remains o n UPS output power.
Static Switch Isolation
The motor-operated system bypass circuit breaker (SBB), wired in parallel with the static switch, automatically
closes in approximately 200 milliseconds after the load is transferred to the bypass power source, removing the
static switch from the power flow.
UPSOutput
Gate
Driver
Sync &
Transfer
Control
Power
Supply
Voltage
Adjust
Alarms
Static
Bypass
Switch
StaticBypassBreaker
Transfer
Control
StaticSwitch
Disconnects
Bypass Line
UPS
System
Modules
To Critical
Load
System
Monitor
DISCONTINUED
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