Static Transfer Switch (STS) is a static electric device that is meant to automatically transfer load from the preferred power source to the backup power source linked to its input in the event of a power outage in the former and vice versa without interfering with the server room load. As the name implies, it is a completely static device devoid of any electromechanical components. Static Switches are capable of transferring load in 1/4th of the cycle time, ensuring uninterrupted power to Datacenter loads. Each phase of a static transfer switch is composed of antiparallel linked thyristors. These thyristors are gate turn-off thyristors (GTOs), which can be activated by a gate signal of positive polarity and deactivated by a gate signal of negative polarity. By managing the on/off switching of these SCRs, a faster load transfer is obtained. Total transfer time is the sum of the time required to detect a disturbance in the source and the time required to transfer load to the alternate source. The schematic for a static transfer switch is illustrated above. Both sources are equipped with thyristor circuits with their own MCCBand Maintenance bypass switch.
Control logic, metering circuit, thyristor circuit, breaker, and bus assembly comprise the static transfer switch. The control logic's function is to continually monitor the current and voltage of both sources and to provide a single gate to the rectifiers. The control logic constantly monitors the voltage and current of the prepared and alternate sources. If the chosen source fails or if there are disturbances such as excessive or insufficient voltage or current in the desired source, the control logic evaluates the status of alternate sources and transfers the load to the source that can provide power within the selectable limit. If both sources are synced with a phase error of less than a certain value, automatic transfer can occur in as little as 6 milliseconds.
Static Transfer Switches are classified into two categories. STS is available in two configurations: single phase and three phase. These are typically rack mountable units with a rating of 16A to 160A. Three phase static switches are available in three or four pole configurations.
Each source in a four-pole system has its own neutral. The neutral is derived separately, and the output neutral is coupled to the neutral of the current source. If the data centre facility has two sources with distinct neutral grounds, a four-pole static transfer switch can be used to obviate the need for a downstream transformer. Three phase static switches are available in capacities ranging from 50A to 1600A.
Several advantages of static transfer switches include the following:
It provides a higher level of quality and thus more reliability. It enables rapid load transfer. The time required to transport a load is only about 4 to 6 milliseconds. Additional protection is provided by an integrated transient voltage surge suppressor. Voltage limit can be selected to safeguard the load against sag, swell, and interruptions. Dry contact supplied for static transfer switch monitoring.