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IB140012EN
Instructional Booklet
Page
38
Effective: March 2021
Operation and Maintenance Manual,
Automatic Transfer Switch Controller, ATC-900
Figure 19. Secondary
Three-source Transfer Switch Arrangement
Primary ATS Connections
•
S1 - Utility
•
S2 - Connected to Load side of Secondary ATS
•
Gen Start - Provides the Primary Out functionality to signal the
Secondary ATS to run its generators.
Secondary ATS Connections
•
(Control power is 24VDC from generator battery)
•
S1 - Generator
•
S2 - Generator
•
Secondary In - Indicates when its generator(s) should be run-
ning.
Secondary Generator Startup/Shutdown Operation
Handling of the Secondary's Gen Start relays for the two genera-
tors will differ based on Preferred Source selection and TDES set-
points on the Secondary controller. The flowcharts at the end of
this document show two different scenarios based on whether or
not a Preferred Source is selected.
In normal operation the Primary is expected to handle TDEC and
the Secondary will remove Gen Start contacts as soon as Second-
ary In is removed.
In the event of a source 1 power failure, (see Figure 19) the Pri-
mary ATS engine start relay closes signaling the Secondary ATS
to start the preferred generator.
Note:
The Secondary ATS requires the DCT module for a DC
power input.
The Primary ATS handles all transfer time delays between the Util-
ity to Generator transfer. If the preferred generator does not start
within the programmed time delay, the Secondary ATC-900 will
signal the non-preferred generator to start. If Preferred is set to
"None", then both generators will start and the Secondary ATS
will transfer to the first generator source available. The second
generator will shut down 5 minutes after the ATC-900 senses the
load is connected to an available source.
Secondary Generator Failure / Transfers
Generators attached to the Secondary controller can fail in two
ways: they can fail to start, or fail while providing power to the
load. Currently, the ATC-900 only reports a Generator Fault
Alarm during starting if a generator fails to start within 90 sec-
onds when in test or plant exercise modes. In these cases, the
Gen Start contact is opened after the 90 second timeout. In non-
test/exercise modes the Gen Start contact is kept closed until
either the generator starts or the reason for attempting to start the
generator is removed (for example, the Secondary In signal is
removed).
Failure of the generator providing power to the load is handled by
reporting a Generator Fail Alarm and transferring to the second
generator with normal time delays for TDES, TDEC, TDEN/TDNE,
etc. The Primary controller sees this only as a failure of S2, and
will react as with any other failure of S2. The Generator Fail
Alarm remains active until either manually cleared or until Second-
ary In is removed and re-asserted to start a new session.
Once a generator fails, the other generator will be kept on until
either the Generator Fail Alarm is manually cleared, or Secondary
In is removed. This is true even if the failed generator restarts and
load is transferred back to it, e.g. if the failed generator is the pre-
ferred source.
Pre Transfer, Post Transfer, Load Sequencing, and Load Shed
The use of Pre Transfer, Post Transfer, Load Sequencing and Load
Shed relays must be carefully considered in view of the dual trans-
fer switches. It may be necessary to mirror these functions on
both controllers and parallel/series relay outputs from both control-
lers to ensure proper control during any transfer. There will be no
special handling of these signals based on Primary/Secondary
setup in Phase 1 ATC-900 controllers.
Engine Test / Plant Exercise
A Primary initiated Engine Test or Plant Exercise will cause the
Secondary to respond just as it does for a normal transfer. A Sec-
ondary initiated Engine Test or Plant Exercise requires Secondary
In to be closed so must be carefully coordinated with the Primary
test setup.
Real Time Clock
The ATC-900 has a battery backed real time clock to track date
and time. The date and time can be set through the front panel
System Info / Set Time & Date screen. It can also be set automat-
ically with an optional Simple Network Time Protocol (SNTP) Cli-
ent Module connected to the ATC-900's Accessory I/O port or by
writing via the Modbus port. There are several settings associate
with the real time clock.
Automatic DST
Sets Automatic Daylight Saving Time (DST) on or off. This follows
the current (2017) North American standard for DST. When this
setpoint is set to ON, time automatically moves ahead by one hour
the second Sunday in March @ 2 a.m.; time automatically moves
back by one hour on the first Sunday in November @ 2 a.m.
Time Sync
Selects the time sync source or turns time sync off.
Two NTP Client module options are currently available: one PLC
based and the other Meter based. Either of these options must be
ordered and properly configured to function correctly. When
selected, the ATC-900 real time clock will be automatically
synced every minute via the SNTP Client Module. Accuracy
between the ATC-900 event logs and the NTP Client time is
within 10 milliseconds.
For complete details regarding setting time via Modbus, see ATC-
900 Modbus Communications Guide IB140006EN.
Time Zone Offset
This setpoint works in conjunction with the time sync options to
accurately display the current time for the location of the ATC-
900 installation. Because SNTP is based on UTC time an offset
must be specified for proper logging and display of local time on
the ATC-900.
Secondary ATS
S2
Primary ATS
S1
Gen 1
Gen 2
Secondary-In
S2
Gen Start
S1
Utility
Load
