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Theory of Operation
16
2.2.5 Static Bypass
A static bypass is an integral part of the UPS System Control Cabinet (SCC). Refer to Figure 7. The
static bypass consists of two reverse-paralleled SCRs (silicon-controlled rectifiers) per phase and
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 sig-
nals and alarm messages and critical bus operating conditions. If the UPS modules can no longer sup-
ply 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 block diagram
Fuse Protection
The static bypass switch path uses two back-to-back SCRs per phase. Each phase is individually pro-
tected by a fuse sized to clear only in the event of a catastrophic fault (with the exception of the con-
tinuous duty static switch used in the SCCC control cabinet). This is a more reliable method than
depending on external protection devices. The fuses are in the circuit to protect the critical bus distri-
bution equipment against catastrophic faults. The static switch SCRs are oversized to easily handle
any current surges that may blow the fuses.
Shorted SCR Monitoring
In all SCCI and SCCC control cabinets, and all SCCT control cabinets 1600A and larger, the static
bypass system has redundant shorted SCR sensing circuits and disconnects. This will prevent UPS
output power backfeed to the distribution system even if two component failures exist simulta-
neously. If a shorted SCR is detected, the static bypass switch is isolated and annunciated at the SCC
control panel, and the critical load remains on UPS output power.
In all SCCT control cabinets smaller than 1600A, the static bypass system is equipped with redun-
dant disconnect circuits that prevent backfeed of lethal voltage to a de-energized bypass input in the
event of a shorted static switch SCR. If the bypass input power is interrupted, the static switch dis-
connect devices will open, preventing backfeed of inverter voltage to the bypass input terminals.
UPS Output
Gate
Driver
Sync &
Transfer
Control
Power
Supply
Voltage
Adjust
Alarms
Static
Bypass
Switch
Static Bypass Breaker
Transfer
Control
Static Switch
Disconnects
Bypass Line
UPS
System
Modules
To Critical
Load
System
Monitor