Static Transfer Switch (STS)

Static Transfer Switches (STS) ensure power continuity by rapidly switching between two independent sources. These solutions maintain power continuity in critical applications, such as Data Centres and industrial facilities. Unlike automatic transfer switches, STS use semiconductors for swift transitions - within a quarter of a cycle in the case of STAYS. Discover how STS work, their advantages and how they differ from other switches to secure your power needs efficiently.

The purpose of STS in critical applications 

Static Transfer Switches (STS) play a vital role in critical applications, providing an alternative power path and adding redundancy. The static transfer switch is indispensable in several sectors where power continuity is essential. Typical applications include:

• Data centres: STS maintain continuous operations, preventing data loss and system downtime while providing increased flexibility to the site.
• Hospitals: critical medical equipment relies on STS for redundant power supply, ensuring patient safety.
• Telecommunication hubs: STS ensure that communication systems remain operational 24/7, without interruption.
• Industrial processes: sensitive machinery is protected from power outages, which could otherwise result in costly downtime.

This static transfer switch operation is crucial in environments where even a brief power interruption can cause significant issues. The reliability of STS in these scenarios underscores their importance in maintaining operational integrity across various critical infrastructures.

Understanding the function of a Static Transfer Switch (STS)

A Static Transfer Switch (STS) is a component ensuring continuous power to critical systems. It works by automatically switching the load between two independent sources. This rapid switching is achieved using silicon-controlled rectifiers (SCRs) which allow the transfer to occur in well below the 20ms that most applications can handle.

The STS is particularly beneficial in environments where power consistency is crucial, such as Data Centres and hospitals, preventing disruption to the load by instantly shifting to an alternate source when the primary source fails or shows signs of instability.

Key benefits of using STS equipment include:

• Fault segregation: faults are isolated, preventing them from affecting the entire system.
• Enhanced power availability is achieved through redundancy.
• Protection is provided against the source of malfunction.  

This technology is a reliable choice for safeguarding sensitive equipment and maintaining uninterrupted operations, even in challenging conditions.

How does a Static Transfer Switch work?

Static Transfer Switches (STS) operate by utilising silicon-controlled rectifiers (SCR) to transfer loads between two power sources. This rapid switching ensures that any disruption is imperceptible, typically occurring within a fraction of a cycle.

With Socomec's STATYS, the SCR drivers integrate both independent and redundant power supplies, incorporating SCR fault sensing for added protection. 

When the primary source experiences a disturbance, the STS immediately shifts the load to the secondary source (see static transfer switch diagram below). This is accomplished without any mechanical components, allowing for swift and reliable transitions. The thyristors play a crucial role by enabling and disabling power flow based on gate signals, ensuring seamless load transfer.

Key components in the STS include:

• Thyristor circuits: based on robust technology, these circuits manage the rapid switching of power sources.
• Control logic: the quality of both sources is checked continuously.
• Sensing circuits: voltage or current fluctuations are detected in order to initiate a transfer.

Socomec has developed an advanced transformer switching management (ATSM) system enabling the management of downstream magnetic loads such as transformers present in PDU. This unique technology protects the current after source transfer on a magnetic load.

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STS (Static Transfer Switch) vs ATS (Automatic Transfer Switch)

Which solution is best suited to which application?

When deciding between an Auto Changeover Switch (ATS) and a Static Transfer Switch (STS), it’s important to consider the specific needs of your application.

• Automatic Transfer Switch (ATS)
• Static Transfer Switch (STS)

Ideal for situations where brief interruptions to power are acceptable. Commonly used in residential and commercial settings where the load can tolerate a short delay in power transfer. The primary source of power is the grid and the secondary could be the grid or a genset. 

The best choice when your application cannot tolerate any interruption to the power supply. Ensuring a swift transfer, often within milliseconds, STS are suitable for sensitive equipment. The primary and secondary power supply sources are typically UPS.

Each type of switch offers distinct advantages based on the criticality and sensitivity of the connected loads.

Key features of rack-mounted Static Transfer Switches

Space optimisation

Rack environments, particularly in Data Centres, have strict space limitations. A rack-specific STS is designed to fit into standard 19-inch racks, utilising vertical and horizontal space efficiently without the need for separate, bulky installations. This is crucial in terms of maximising the density of equipment within the available rack space.

Simplified integration

Rack-specific STS units are typically designed to integrate easily with other rack-mounted equipment, such as servers, power distribution units (PDUs) and small IT racks. They are engineered to be easily installed in standard rack formats, streamlining the setup process and reducing the need for additional configuration or infrastructure changes.

Front accessible bypass allows for concurrent maintenance by keeping the load supplied while the STS power brick is extracted.  

Key features of cabinet Static Transfer Switches

Space constraints and integration flexibility

• Custom fit for unique spaces
• Non-rack environments

The cabinet version allows for standalone installation, providing flexibility for facilities with different layout requirements.  The wide range of STATYS equipment means that all types of layout requirements can be met, from being adjacent to the IT row to being located next to the UPS.

Cabinet versions can be installed in environments where there are no standardised racks, such as industrial plants, hospitals or within other critical infrastructure, providing more options for system placement.

Scalability

• Future expansion

The use of a “catcher” type distribution offers space for future expansion, making it easier to integrate additional rooms or upgrades in order to handle larger power capacities without requiring an overhaul of the existing infrastructure.

Maintenance

• Isolated components

Cabinet versions are built around redundant components with internal segregation reducing the risk of fault propagation and simplifying troubleshooting.

Compliance with specific regulations

• Safety and compliance

In some industries, specific regulations regarding power equipment installation, safety standards or spatial requirements (such as clearance around the unit) must be met, and a cabinet version may provide the flexibility that’s needed in order to comply with these standards.

In today's world, where power continuity is vital and directly impacts competitiveness, Socomec's STATYS range truly stands out. With over 35 years of experience and millions of operational hours, Socomec has continuously enhanced its offering. Now, the fourth generation of STATYS ensures uninterrupted power availability for applications ranging from 32 to 1800 A.