In the last issue, we discussed the types of automatic transfer switches (ATS) and took a close look at the open-transition ATS. This continuation of the ATS coverage examines delayed transition transfer switches (DTTS). One of the most important considerations of the design of the emergency power system is selecting the right equipment for the right application. Getting this right will add many years to the viability of your design.
DTTS provide an extended open period between sources for a number of different reasons. The design mimics that of the closed transition ATS with a programmable open time between sources. The most common issue is the inrush current typically associated with large inductive loads (transformers, motors, etc). On a cold start, large inductive loads require substantial current in order to establish a magnetic field. This can be eight to times the normal running current. Since this is normal and not considered a fault, protective devices need to ignore this transient. During power switching operations, we must meet the inrush current created as a result of an “open” transition between two hot sources. When power is removed from large inductive loads, their windings give up residual power briefly as their magnetic fields collapse. This is known as back EMF (electromotive force). We also have to accommodate the phase angle of the alternate source. Transferring too quickly between two hot sources and without considering the phase angle of the alternate source results in massive current.
The DTTS has two separate operating mechanisms with a programmable delay or open time between “normal” and “emergency.” Programmed with an appropriate open window, the DTTS allows the field of a disconnected load to collapse and back EMF to dissipate before re-connecting to the alternate source, leaving only normal black start inrush.
DTTS units are available with the isolation bypass feature, as with many other types of ATS. However it’s important to note that the isolation bypass feature is a manually operated unit, which does not incorporate the ability to delay transition between the sources. It is still possible to connect the isolation bypass feature to the critical load without interruption; however should it become necessary to manually transfer between two hot sources while using the isolation bypass feature, the operator must observe caution and open the source breakers, transfer the bypass MTS, and reclose the appropriate breaker after the transfer is completed. As with any switching operation, the use of appropriate PPE is required.