System Description
Ctrl Nr: PM375118-004
15
1.4
Transfer Process: Power Switching Components
There are three sets of Molded Case Circuit Breakers (MCCBs) or Molded Case Switches (MCSWs) controlling
whether the system is in Normal or Bypass Mode and two sets of SCRs, which provide power switching for
transfers:
•
Source 1 MCCB and Source 2 MCCB are breakers from the two power sources to the SCRs.
•
SCR 1 and SCR 2 open or close to control which source powers the load.
•
Bypass 1 MCCB and Bypass 2 MCCB are used to shunt power around STS logic and power switching
elements for maintenance.
•
Isolation 1 MCSW and (optional) Isolation 2 MCSW isolate components for maintenance during Bypass
Mode. The second optional isolation MCSW provides redundancy.
Figure 6 Normal Mode MCCB/MCSW and SCR Configuration
In Normal Mode operation (Figure 6), the Monitor controls the transfer process:
•
The MCCBs for Source 1 and 2 are both closed, allowing a power path from either source, hence
allowing the load to be automatically transferred from one source to the other.
•
The Isolation MCSWs are closed to provide a power path for either source.
•
System logic opens or closes SCR 1 or SCR 2 to transfer the load rapidly from one source to the other.
•
Bypass 1 and 2 MCCBs are both open while the bypass is unused.
The Monitor logic continuously monitors the two power sources using its sensing algorithms and controls
transfers, using the POG or VSS algorithm, as specified by the user. The Monitor is itself continuously monitored
In Normal Mode, breakers
(MCCBs and MCSWs) are fixed
in the open/closed configuration
shown here.
-Source (Input) 1 and 2 MCCBs
are both closed, allowing either
source to power the load.
-Bypass breakers are both open.
-The Isolation (Iso) MCSWs are
closed to allow a power path from
either source.
In Normal Mode,
-SCRs determine the
active power path under
control of system logic. In
this illustration,
-SCR 1 is closed (active
path).
-SCR 2 is open (inactive
standby path).
One-line Diagrams
are from the Home
screen.
In the illustration to the left,
- Source (Input 2) is powering the load.
- SCR 2 is closed, SCR 1 open.
- Isolation switches are closed to allow a
power path from either source.
Note: Only (1) Isolation Switch is required;
(2) Isolation Switches provide redundancy.
