TSC 900 TRANSFER SWITCH CONTROLLER
PM 151 REV 1 15/10/08 Thomson Power Systems
3.4. AUTOMATIC SEQUENCE OF OPERATION
3.4.1. OPEN TRANSITION TRANSFER
Note: For specific device settings and ranges, refer to Section 6 - Factory Default
Programming.
Under normal 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 generators
voltage and frequency levels. Once the generator voltage and frequency rises above
preset values, a warm up time delay will be initiated. Once the warm up timer expires,
the transfer to utility supply signal will be removed (i.e. contact opening) and the
transfer to generator supply signal (contact closure) will be given to the transfer switch
mechanism. 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 transfer to generator supply
signal will be removed (contact opening), the transfer to utility supply signal (contact
closure) will be given to the transfer switch mechanism. The load will then be
transferred from the generator supply back to the utility supply. During the utility re-
transfer sequence a neutral position delay circuit can be employed which will cause the
transfer mechanism to pause in the “neutral position (i.e. with both transfer power
switching devices open) for the duration of the neutral delay timer setting, once the
time delay expires, the re-transfer sequence will be completed.
An engine cooldown timer circuit will be initiated once the load has successfully re-
transferred back onto the utility supply. Following expiry of the cooldown delay period
the engine start signal will be removed (remote start contact opened) to initiate
stopping of the generator set.
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