Static Transfer Switch (STS)

What Is STS?

STS(Static Transfer Switch), is an automatic static switching equipment designed to transfer critical loads between two independent AC power sources without interruption or with a transfer time of less than a cycle (20ms).

The STS consists of Silicon controlled rectifiers (SCR’s), control & sensing circuits to monitor the incoming sources and transfer the critical loads connected to the secondary source when the primary source fails. The schematic of an STS is shown below (figure 1)



Figure 1 Schematic Drawing of STS

Reliability diagram of STS

The availability of the STS is based on the major components which are used like Static switch, its driver, power supply and the common circuits which include sensing circuit, microcontroller/DSP, cooling system as shown in figure 2.



Figure 2 Reliability Diagram of STS

The static switch, driver PCB’s and the power supply are parallel components. The common circuits are series components. The overall availability of STS is shown in the below table.

The table also shows the availability of an STS without any specific internal redundancy and with an assumption of natural cooling to achieve the highest availability. On the other hand, if the STS design requires a forced air cooling the reliability comes down as the fans are consumables and are subject to failure which can impact the performance of STS.



Table 1 Availability of STS

Improving reliability of STS

From the table referred in the previous page, it is evident that the chances of the failure is low, which increases the overall availability of STS.

It is also possible to build up an internal redundancy in the power supply with an criss-cross connection of the power supply (as shown in Fig.3)to improve the availability of power supply



Figure 3 Criss-Cross Connection of Power Supply

The static switch consists of an inbuilt manual bypass (as shown in fig 1) to ensure the availability of the power for the critical loads connected to the STS.

The availability of STS can also be improved by reducing the MTTR, by using hot swappable STS modules instead of conventional monolithic construction.

Comparison of reliability of power distribution system with UPS & STS

In an conventional UPS configuration with N+1 configuration, where N=2, we put in place a parallel configuration with 3 UPSs in parallel and an common output from the parallel bus is taken and provided as a input to the critical loads.

Although this configuration provides the redundancy at UPS level, there is still a possibility of a single point of failure, which is the common parallel bus (a rarest scenario).

Disadvantages of the configuration includes

  • o Concurrent maintenance is not possible
    o Critical parallel bus is not a fault tolerant design, should there be fault generated down stream, the same can be propagated to the critical bus impacting the availability for other critical loads.
    o Single point of failure in parallel bus

These above mentioned disadvantages can be overcome by optimising the system design using an STS and the common parallel bus is replaced with a three independent STS with a criss-cross connection as shown in the below table.



Table 2 Reliability Comparison of N+1 UPS with and without STS

This configuration helps us to have

  • a concurrent maintenance of the distribution system as the UPS can be removed without disturbing the load.
  • compartmentalisation of fault in the downstream distribution of the load is possible as the STS doesn’t allow the fault to transfer(example a fault in load connected to static switch C, will impact only the loads connected to that specific static switch).
  • no single point of failure as we migrate to multiple N configurations instead of conventional N+1 configuration.

The above schematics can be used for any critical applications not limited to

  • Data centre
  • Intensive care units in Hospitals
  • Operation Theatres in Hospital
  • Signalling & communications for Metro Rails, railways
  • Critical IT operations
  • E-commerce businesses
  • Telecommunication centres
  • Network operations centres
  • Optic Fibre nerve centre
  • Process control
  • Airport security, air traffic centre system
  • Commercial buildings
  • Power generation plants
  • Power quality and industrial parks
  • Automated manufacturing
  • R&D labs
  • Semiconductor industry

There are various configuration of UPS is possible as shown below in figure


Availability improvement with STS

It is also evident from the table 2, that the implementation of an STS will improve the MTBF and the availability of the critical loads.

In addition to the improved availability of power for the critical loads, the implementation of STS will also provide additional advantages like

  • Fault Discrimination/SegregationCompartmentalisation of fault, avoid propagation of fault
  • Improved utilisation factor
  • Optimise the capacity of UPS based on the load and improve the loading levels
  • Better Efficiency
  • Better PUE
  • Lower Operational Cost
  • Flexibility of Installation ->adapt the installation based on the load levels
  • Concurrent Maintenance -> helps to maintain or repair the distribution system without impacting a downtime on the connected loads.