TSC 900 TRANSFER SWITCH CONTROLLER
PM 151 REV 1 15/10/08 Thomson Power Systems
3.4.2. CLOSED TRANSITION TRANSFER
For transfer switches equipped with the closed transition transfer option (i.e. ATS
Model Code Digit #13 “Operation Type” 3 or 4), the TSC 900 is configured to provide
additional logic for this application. When the TSC 900 controller receives an input
signal for Closed Transition Transfer Mode, the TSC 900 is configured to operate as
follows:
Under normal closed transition operating conditions, the transfer switch operates
automatically during a failure and restoration of utility power and does not require
operator intervention.
When utility supply voltage drops below a preset nominal value on any phase, an
engine start delay circuit will be initiated. Following expiry of the engine start delay
period an engine start signal (contact closure) will be given.
Once the engine starts, the transfer switch controller will monitor the generator voltage
and frequency levels. When the generator voltage and frequency rises above preset
values, a warm up time delay will be initiated. When the warm up timer expires the
transfer to utility supply signal will be removed (logic contact(s) opening) and the
transfer to generator supply signal (logic contact(s) closure) will be given to the transfer
switch Power Switching Devices. The load will then transfer from the utility supply (i.e.
opening the utility power switching device) to the generator supply (closing the
generator power switching device) to complete a break-before-make open transition
transfer sequence.
The generator will continue to supply the load until the utility supply has returned and
the retransfer sequence is completed as follows: When the utility supply voltage is
restored to above the preset values on all phases, a utility return delay circuit will be
initiated. Following expiry of the utility return timer, the utility power-switching device
will close when it is in synchronism with the generator supply. If the transfer switch is
supplied with a Fast (Momentary) Closed Transition transfer control option, the
generator power switching device will immediately trip within ~100 milliseconds after
the utility power switching device closes to complete the “make-before-break” re-
transfer sequence. If the transfer switch is supplied with a “Soft-Load” Closed
Transition transfer control option, the generator power switching device will remain
closed for a longer time period to allow a soft-load power transfer sequence to be
completed via external loading controller. The generator power switching device will
then trip open to complete the “make-before-break” re-transfer sequence.
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